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SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY NUMBER 129

Two New Species and a New Subgenus of Lucinidae (Mollusca: Bivalvia), with Notes on Certain Aspects of Lucinid Phylogeny

Joseph C. Britton, Jr.

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON

1972

ABSTRACT

Britton, Joseph C. Two New Species and a New Subgenus of Lucinidae (Mollusca: Bivalvia), with Notes on Certain Aspects of Lucinid Phylogeny. Smithsonian Contributions to Zoology, number 129, 19 pages, 6 figures, 1972.–Two new western Atlantic species of Lucinidae, Lucina (Pleurolucina) hendersoni, new species, and Parvilucina (Bellucina) rehderi, new species, are described and figured. A new subgenus, Radiolucina (type-species, Phacoides amiantus Dall, 1901), of the genus Parvilucina is proposed. Certain aspects of the shell morphology of several groups of Lucinidae with similar hinge structure are compared. The nature of radial ornamentation in the subgenus Pleurolucina appears to differ significantly from that of Parvilucina or Radiolucina, suggesting the former should be considered distinct from the latter groups. Certain aspects of the shell morphology of Bellucina is demonstrated to be sufficiently distinct from that of species such as Parvilucina amiantus (Dall, 1901) to justify the establishment of a new subgenus, Radiolucina, for the latter. Several additional aspects of lucinid phylogeny are considered.

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Official publication date for each title is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year.

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Library of Congress Cataloging in Publication Data

Britton, Joseph C.

Two new species and new subgenus of Lucinidae (Mollusca: Bivalvia)

(Smithsonian contributions to zoology, no. 129)

Bibliography: p.

1. Parvilucina rehderi. 2. Lucina hendersoni. 3. Lucinidae.

I. Title. II. Series: Smithsonian Institution. Smithsonian contributions to zoology, no. 129

QL1.S54 no. 129 [QL430.7.L8] 591′.08s [594′11] 72–3864

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington, D.C. 20402 - Price 35 cents (paper cover)

Two New Species and a New Subgenus of Lucinidae (Mollusca: Bivalvia), with Notes on Certain Aspects of Lucinid Phylogeny

Joseph C. Britton, Jr.

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Joseph C. Britton, Department of Biology, Texas Christian University, Fort Worth, Texas 76129.

Introduction

Within the family Lucinidae (Mollusca: Bivalvia) there is a large, complex assemblage of species sharing several similar morphological characteristics, the most noticeable being the similarity of hinge structure. Because previous authors have assigned these species to several different supraspecific categories, each of the supraspecific taxa discussed herein must be defined. Table 1 contains a list of the taxa and their type-species.

Except for minor modifications, all of the groups listed in Table 1 possess a full complement of lucinid dental elements. The following Bernard dental formula (see Cox, et al., 1969:N56, for an explanation of the Bernard formula) characterizes the hinge of this assemblage:

In some species of each of the groups except Lucina, sensu stricto, and Bellucina, the right anterior cardinal tooth (3a) fails to develop or is obscured by encroachment of the lunule; whereas in some groups (particularly Bellucina) the left dorsal lateral teeth (AIV and PIV) may be reduced. Considering the total variation of dentition within the entire family, these variations are slight and suggest that these groups are more closely related to each other than to any other group (s) of Lucinidae (e.g., Codakia has weak or obsolete posterior laterals, Miltha lacks lateral dentition entirely, and Anodontia is edentulate).

Previous authors (particularly Dall, 1901; Chavan, 1937, 1938, 1969; and Bretsky, 1969) have recognized the basic similarity of dentition among the groups listed in Table 1, but their phylogenetic and systematic interpretations vary according to the degree of emphasis each author places upon (1) subtile variations of hinge dentition, (2) other aspects of shell morphology, or (3) a combination of these two.

Although there were several earlier works which purported to be revisionary, Dall (1901) should be considered the “first reviser” of the Lucinidae. He recognized the tremendous diversity of the lucinids and attempted to demonstrate this knowledge nomenclatorially. Unfortunately, many of his nomenclatorial opinions were to confuse rather than to clarify lucinid phylogeny. Dall recognized only six genera of Lucinidae: Lucina Bruguière, 1797 [type-species considered by Dall to be Lucina edentula (Linnaeus, 1758), by subsequent monotypy, Lamarck, 1799]; Loripes Cuvier, 1799; Codakia Scopoli, 1777; Myrtea Turton, 1822; Phacoides Blainville, 1825; and Divaricella von Martens, 1880. He subdivided each of these genera into several subgenera and sections. He considered all of the species mentioned in Table 1 to be within his largest and most inclusive genus, Phacoides (see Table 2).

TABLE 1.–Supraspecific taxa of Lucinidae receiving primary consideration in this publication

The most important aspect of this first revision of the Lucinidae is the phylogenetic relationships which can be inferred from it. Assuming Dall’s “subgenera” to roughly correspond to Bretsky’s (1971) higher “phenon groups,” Dall recognized two basic lineages: the Here lineage (corresponding to the Lucina lineage of this paper) including the species Phacoides richtofeni, P. pensylvanica, P. leucocyma, and P. trisulcata; and the Parvilucina lineage (the Parvilucina, sensu lato, lineage or the Parvilucina-Radiolucina-Bellucina complex of this paper) including the species Phacoides tenuisculpta, P. semperiana, and P. amiantus. As will be shown, there is considerable merit to an arrangement of this type.

Chavan (1937, 1938) obviously drew heavily from Dall’s classification of the Lucinidae, but his interpretation of the family was influenced by two new opinions: that Phacoides was a term introduced in the vernacular and was not a name available for zoological nomenclature, and that Lucina jamaicensis (Spengler, 1784)¹ (=Tellina pectinata Gmelin, 1791) was the type-species of Lucina according to subsequent designation. The problem of the type-species of Lucina is a complex nomenclatorial matter. Except as indicated here it is beyond the scope of the present paper. (For additional information, see Chavan, 1937, 1938, 1952; Bretsky, 1969; Britton, 1970.) Thus, instead of the groups of Table 1 being considered of the genus Phacoides, Chavan placed them in the genus Linga. He affirmed that phylogenetic relationships between and among these taxa were very difficult to understand, and proposed a taxonomic solution whereby each of the major lineages within his genus Linga were subgenera with more-or-less equal status. Like Dall, Chavan recognized all of the groups of Table 1 to be more closely related to each other than to other Lucinidae, a fact which becomes less obvious in his more recent classification (Chavan, 1969). The details of both Chavan classifications are given in Table 2. Note that Chavan (1937, 1938) considered Phacoides amiantus Dall, 1901, to be a member of the subgenus Pleurolucina rather than Bellucina to which it was originally referred by Dall. Much more will be said of this matter shortly.

TABLE 2.–Comparison of supraspecific assignments by previous authors of certain species of Lucinidae

In the more recent Chavan classification (1969), several groups previously considered subgenera are elevated to genera, including Here and Parvilucina (Table 2). Pleurolucina and Bellucina are considered subgenera of the genus Linga; whereas Cavilinga is a subgenus of the genus Parvilucina. Hence, Chavan (1969) does not consider Bellucina to be as closely related to Parvilucina as it was originally considered by Dall (1901).

A few months before publication of Chavan’s more recent classification, Bretsky (1969) completed a doctoral dissertation on phenetic and phylogenetic classifications of the Lucinidae. Bretsky’s systematic treatment of the Lucina, sensu lato, complex is quite similar to that of Chavan (1937, 1939), as she considers each of the groups subgenera of a single taxon (Table 2). Yet her classification differs in several details from both of Chavan’s classifications. For example, Bretsky gives a very convincing argument (accepted in this paper) favoring Schumacher’s (1817) subsequent designation of Venus pensylvanica Linnaeus, 1758, as the first valid type-species designation of Lucina. She also rejects Chavan’s contention that Phacoides amiantus Dall, 1901, is a Pleurolucina, placing it in the subgenus Bellucina. Bretsky indicates, however, that specimens of the type-species of Bellucina (i.e., specimens of Lucina semperiana Issel, 1869) were not available for her examination. Hence, Bretsky’s concept of Bellucina is based on P. amiantus and its close relatives rather than upon the type-species. Some of these data have subsequently been published (Bretsky, 1971).

While preparing a monograph of the Western Atlantic Lucinidae, I discovered two new species which help to clarify relationships among the taxa of Table 1. One of the new species also has been influential in the decision to designate a new subgenus, although it is not of the new subgenus itself. The new species and new subgenus are described below, followed by a discussion of their significance and relationship to the taxa of Table 1.

Acknowledgments

This paper is based on data obtained in part during preparation of a doctoral dissertation for The George Washington University, under the direction of Dr. Harald A. Rehder, National Museum of Natural History, Smithsonian Institution. I would like to express my gratitude to Dr. Rehder for many valuable hours of consultation which helped make this work possible, and for numerous helpful suggestions during preparation of the manuscript. I am also indebted to Dr. Joseph Rosewater, also of the National Museum of Natural History, for additional guidance and criticism.

Lucina (Pleurolucina) hendersoni, new species

FIGURES 1, 2, and 6c

TYPE.–Holotype, USNM 503399; type-locality, off English Harbor, Antigua in 120 fms.

DESCRIPTION.–General Form: Shell attaining 12.5 mm in length and 12.3 mm in height, more or less trigonal, equivalve, inequilateral with beaks posterior to midlength, valves moderately inflated; posterodorsal margin gently curving from beak to beyond posterior termination of hinge plate; posterior margin gently curving to marginal termination of posterior sulcus, there becoming indented; posteroventral, ventral, and anteroventral margins smoothly curving (see p. 6); anterior margin slightly projecting, indented at marginal termination of dorsalmost anterior sulcus; anterodorsal margin slightly arching above hinge plate.

Beaks, Umbos: Beaks coiled under umbos, posterior to midlength of shell. Umbos coiled, prosogyrate, distinctly projecting above body of shell.

Sulci, Folds, Ridges: Posterior sulcus strong, extending from beak to ventral margin; posterior dorsal area bearing sculpture as on body of shell. Three or four anterior sulci variably expressed; dorsalmost anterior sulcus strongly impressed, extending from beak to anterior margin, forming distinct anterior dorsal area or pseudolunule; second or next posteriormost sulcus relatively strong on dorsal half of valve, but becoming weak or obscure toward margin; third sulcus restricted to umbo, centrally located, crossing few strongly elevated concentric lamellae; fourth sulcus extremely obscure, occuring only on dorsal portion of umbo between third sulcus and posterior sulcus.

FIGURE 1.–Lucina (Pleurolucina) hendersoni, new species, holotype, USNM 503399: a, external aspect of left valve; b, internal aspect of left valve. Length, 12.3 mm.

FIGURE 2.–Paratype specimens of Lucina (Pleurolucina) hendersoni, new species, USNM 712452. Note how ventral marginal outline becomcs less irregular with increase in size. Length of (e), 10.2 mm; others to scale.

Lunule, Escutcheon, External Ligament Area: Lunule very small, slightly depressed, located immediately ventral of and extending slightly posterior to beaks, essentially symmetrical, not projecting. Escutcheon absent. Insufficient material to describe ligament suture or nature of external ligament area.

Ornamentation: Except for sulci noted above, radial ornamentation absent; concentric sculpture consisting of several widely spaced, distinctly erect, concentric lamellae, fewer than one lamella per millimeter at a point between 4 and 5 millimeters ventral to umbo, dorsal edge of each lamella sharply formed; secondary concentrics present on interspaces between primary lamellae, low, feeble, numerous in each interspace; primary lamellae interrupted by posterior and first anterior sulcus, interrupted on dorsal half of shell by second anterior sulcus, and interrupted on umbo by third anterior sulcus.

Internal Shell Morphology: Hinge plate moderately thickened; ligament opisthodetic, moderately inset, relatively short; dorsal lamellar layer very thin, extending from beak to half-way to anterior termination of posterior laterals, in contact with ventral fibrous layer along entire length of latter; fibrous layer separated from posterior laterals by undifferentiated portion of hinge plate almost equal to length of posterior laterals, moderately thickened centrally, separated from ventral margin of hinge plate by undifferentiated region narrower than width of ligament area.

Dentition of right valve including anterior lateral tooth (A III), opisthocline, narrow, sharply pointed, distinctly protruding, separated from anterodorsal margin by shallow groove, ventral portion of tooth on distinct ridge of hinge plate and with cusp to receive left ventral anterior lateral; hinge plate narrowing toward cardinal area; anterior lateral separated from anterior termination of lunule; lunule not jutting, majority of lunule under and posterior to beaks; anterior cardinal tooth (3a) small, feeble, slightly elevated, essentially orthocline, positioned on ventral portion of hinge plate and slightly posterior to anterior termination of lunule, separated from posterior cardinal by small narrow socket to receive left anterior cardinal; posterior cardinal tooth (3b) moderately enlarged, prosocline, bifid, situated on ventral margin of hinge plate, anterior portion projecting slightly ventrad, separated from ligament area by broad shallow socket to receive left posterior cardinal; posterior lateral tooth (P III) prosocline, moderately elongate, distinctly elevated but less so than right anterior lateral, separated from posterodorsal margin by narrow groove, ventral surface with distinct cusp to receive left ventral posterior lateral.

Dentition of left valve including two orthocline anterior lateral teeth with ventral member somewhat anterior to dorsal member; dorsal anterior lateral tooth (A IV) not strongly elevated, separated from dorsal margin by broad shallow groove, separated from ventral anterior lateral by broad, deep socket to receive right anterior lateral; left ventral anterior lateral tooth (A II) bluntly pointed, distinctly projecting, located on ventral portion of hinge plate posterior and immediately dorsal to dorsal termination of anterior adductor muscle impression; anterior laterals separated from cardinal area by undifferentiated portion of hinge plate extending approximately three times length of left dorsal anterior lateral; lunule as in right valve; anterior cardinal tooth (2) essentially orthocline, distinctly projecting, located on ventral projection of hinge plate, not elongate in dorsoventral axis, dorsal termination of tooth separated from ventral rim of lunule by narrow depression, apex of tooth lying almost ventral and slightly anterior to anterior termination of posterior cardinal, separated from latter by relatively deep, broad socket to receive right posterior cardinal, less remote from beak than corresponding tooth in P. leucocyma; posterior cardinal tooth (4b) prosocline, lamelliform, distinctly elevated, separated from ligament area by narrow undifferentiated portion of hinge plate; posterior lateral teeth prosocline; dorsal posterior lateral tooth (P IV) very weak, slightly elevated, separated from ventral posterior lateral by elongate socket to receive right posterior lateral; left ventral posterior lateral tooth (P II) strongly elevated, elongate, apex bluntly rounded and almost appearing flattened, posterior termination of tooth immediately dorsal to dorsal termination of posterior adductor muscle impression.

Anterior adductor muscle impression elongate, more or less typically lucinoid, somewhat L-shaped, pallial line junction slightly dorsal to midlength of adductor impression, dorsal and ventral portions of impression undifferentiated, anterior pedal retractor impression small, separated from anterior adductor impression, located immediately above dorsal termination of latter and immediately below anterior termination of anterior laterals; pallial line typical, relatively narrow; posterior adductor muscle impression relatively small, ovate, bluntly pointed dorsally, rounded ventrally, pallial line junction slightly above distalmost portion of impression. Line of gill attachment and pallial blood vessel impression faintly indicated. Internal ventral margin finely crenulate.

Soft Parts: Neither living specimens nor specimens with preserved soft parts were available for study.

Color of Shell: All fresh specimens examined possessed uniformly chalky white shells.

Measurements: There are no paired valves of this species available for study. Measurements below are in millimeters.

VARIATION.–With very few specimens available for examination, only a few remarks on variation are possible here. Marginal outline changes noticeably with the growth of the shell (Figure 2). In younger shells (6 mm or less in length) there are two distinct marginal indentations: one on the posteroventral margin at the termination of the posterior sulcus, and the other on the anteroventral margin at the termination of the second anterior sulcus. As the shell increases in size, the second anterior sulcus becomes progressively weaker until it is no longer impressed in the ventral portion of the shell. Concurrently the anteroventral indention of the shell margin becomes less noticeable. In the adult (8 mm or more) this area is smoothly curving without a trace of indentation.

The spacing of primary concentric lamellae on the shell exterior of the holotype is representative of the condition observed in most specimens available for study. In one valve (a single specimen from off La Habana, Cuba) these lamellae are more numerous than on the holotype and considerably more closely spaced (about two per millimeter). The lamellae of this specimen does not approach even closely the high density condition of the lamellae of L. leucocyma, but it certainly demonstrates that a potential for more numerous, more closely spaced primary concentrics exists within the lineage.

REMARKS.–This species is named in honor of J. B. Henderson, Jr., a collector extraordinaire during the early years of this century.

L. hendersoni is a species linking Lucina, sensu stricto, with Pleurolucina. The nature of the external sculpture is very similar to that of Lucina pensylvanica, and L. hendersoni also possesses a distinct pseudolunule or anterior dorsal area quite similar to that of the Lucina, sensu stricto, assemblage. The secondary sulcations, however, are decidedly as in Pleurolucina, even if they are incompletely indicated in the adult. These sulcations are discussed in considerable detail below. The nature of the shell interior further establishes the affinities of L. hendersoni with Pleurolucina. This species and L. leucocyma are almost identical in all characters of internal shell morphology except in the left valve the anterior cardinal of L. hendersoni is less ventrally jutting on the hinge plate, less remote from the beak, and the socket between this tooth and the posterior lateral is shallower and narrower than in L. leucocyma. L. hendersoni shares many common characteristics with the fossil Lucina (Paslucina) follis Olsson, 1964, from the early Miocene of Ecuador.

RANGE.–Based on the present records for this species, L. hendersoni is assumed to occur in moderate depths from Cuba to the Lesser Antilles. It appears to be quite uncommonly collected.

MATERIAL EXAMINED.–CUBA: off La Habana in 127 fms (USNM). LESSER ANTILLES: off English Harbor in 120 fms, off Paynes Bay Church in 50 fms, off Telegraph Station in 50 to 60 fms, and off Pelican Is. in 100 fms, Barbados (all USNM).

Parvilucina (Bellucina) rehderi, new species

FIGURE 3

TYPE.–Holotype, USNM 208255; type-locality, Brazil.

DESCRIPTION.–General Form: Shell attaining 4.2 mm in length and 4.2 mm in height, subtrigonal, equivalve, slightly inequilateral with beaks posterior to midlength, valves moderately inflated. Posterodorsal margin slightly curving from beak to point near posterior termination of hinge plate, thereafter passing rather abruptly ventrad; posterior margin straight; posteroventral margin rather sharply angular; posterior margin gently curving to about midlength of shell, thereafter arc of curve becomes sharper; anteroventral margin somewhat produced but smoothly curving; anterior margin dorsoventrally straight for short distance; anterodorsal margin arching above anterior laterals.

FIGURE 3.–Parvilucina (Bellucina) rehderi, new species, holotype, USNM 208255: a, external aspect of the left valve; b, external aspect of right valve; c, internal aspect of left valve; d, internal aspect of right valve; e, dorsal aspect of paired valves. Length, 4.2 mm.

Beaks, Umbos: Beaks small, depressed, adjacent but not touching. Umbos moderately projecting above body of shell.

Sulci, Folds, Ridges: Posterior sulcus broad and widening ventrally, sufficiently depressed to indicate posterior dorsal area; posterior dorsal area distinct, tending to be elevated marginally. Anterior sulcus poorly impressed; anterior dorsal area poorly differentiated.

Lunule, Escutcheon, External Ligament Area: Lunule distinct, approximately as long as wide, inserted slightly beneath beak, extending from there to point immediately posterior to posterior termination of anterior laterals, depressed, slightly unsymmetrical with portion on right valve projecting and portion on left valve recessed. Escutcheon absent. External ligament area quite short, about one-half width of lunule.

Ornamentation: Dominant sculpture concentric, lamellae elevated, thickened, moderately spaced, interspaces between lamellae at least width of lamellae and usually broader, lamellae on ventral portion of valve usually more elevated than those on upper third of valve, lamellae often appear scalloped by forming nodules in positions adjacent to elevated regions of radial sculpture; radial ribs present but poorly distinguished, best developed in interspaces between concentric lamellae particularly on dorsal third of valve, appearing as poorly elevated columnar elements between concentrics, rarely sufficiently distinct so as to appear to cross concentrics although ventral nodes on some concentric lamellae may be fused with some ribs; ribs broad, separated by very shallow, narrow interspaces; ribs tend to become progressively less distinct toward ventral margin or to lie primarily upon dorsal surface of the broadest concentric lamellae instead of being located only in interspaces between lamellae. Very low scales formed by local elevations of concentric lamellae tend to occur on posterior dorsal area.

Internal Shell Morphology: Hinge plate noticeably thickened; ligament opisthodetic, slightly inset, quite short, extending from beak to approximately midlength between beak and apex of posterior laterals, extending only slightly beyond posterior termination of cardinal plateau; material insufficient to determine nature of ligament layers.

Dentition of right valve including anterior lateral tooth (A III), large, bulbous, prominently elevated, located immediately in front of and ventral to anterior termination of lunule, resting on ventrally projecting ridge of hinge plate, separated from anterodorsal margin by broad groove, possessing cusp on ventral surface to receive left ventral anterior lateral; right anterior cardinal tooth (3a) present, opisthocline, small, slightly crowded by encroachment of lunule, more or less lamelliform, extending from point ventral and anterior to posterior termination of lunule and passing anteroventrally, not in contact with ventral margin of hinge plate, separated from ventral surface of lunule by shallow groove, contacting dorsal termination of right posterior cardinal but separated from remainder of latter by triangular socket receiving left anterior cardinal; right posterior cardinal tooth (3b) prosocline, large, well elevated, separated from ligament area by elongate socket receiving left posterior cardinal; right posterior lateral tooth (P III) elongate, anteriorly broad, posterior portion centrally excavated to receive left posterior lateral, separated from posterodorsal margin by broad groove, separated from cardinal area by undifferentiated portion of hinge plate less than length of posterior lateral.

Dentition of left valve including two anterior lateral teeth; dorsal anterior lateral tooth (A IV) extremely feeble, essentially obsolete; ventral anterior lateral tooth (A II) bluntly pointed, moderately elevated, occupying ventrally projecting ridge of hinge plate, located noticeably in front of and ventral to dorsal anterior lateral, separated from latter by deep socket receiving right anterior lateral; left cardinal teeth lamelliform; anterior cardinal tooth (2) orthocline, noticeably elevated, extending from posterior termination of lunule toward but not reaching ventral termination of hinge plate, separated from greater portion of lunule by narrow socket receiving right anterior cardinal, separated from left posterior cardinal by broad triangular socket receiving right posterior cardinal; left posterior cardinal tooth (4b) prosocline, extending from near posterior termination of lunule toward ventral margin of hinge plate, essentially adjacent and in contact with ligament area; two posterior lateral teeth; dorsal posterior lateral tooth (P IV) extremely feeble, essentially obsolete; ventral posterior lateral tooth (P II) bulbous, elevated, positioned on ventrally projecting ridge of hinge plate, located behind and below dorsal posterior lateral, separated from latter by broad, elongate socket receiving right posterior lateral.

Anterior adductor muscle impression longer than wide but very short, pallial line joining anterior adductor impression near ventral termination of latter; anterior pedal retractor muscle impression small, separate from anterior adductor muscle impression, ovate, located on lower surface of anterior lateral tooth of each valve; posterior pedal retractor impression obscure. Line of gill attachment and pallial blood vessel impression not indicated on specimens available for study. Ventral margin dentate, almost appearing fluted.

Soft Parts: Neither living specimens nor specimens with preserved soft parts were available for study.

Color of Shell: All specimens examined possessed uniformly chalky white shells.

MEASUREMENTS.–Including the holotype, there were 26 left valves and 30 right valves of the new species P. rehderi available for study. Measurements (in mm) given below are of a representative series selected from this material.

VARIATION.–The most noticeable variation observed among the specimens available for study concerns the thickness and spacing of concentric lamellae and the degree to which radial elements of sculpture are developed and expressed on the shell exterior. Radial sculpture never approaches the degree of expression demonstrated by that of Parvilucina (Bellucina) semperiana.

In Parvilucina rehderi, the posterior sulcus tends to be more deeply impressed in some specimens than in others. In those specimens with a more deeply impressed posterior sulcus the angles formed along the posterior margins of the shell tend to be more acute.

The degree to which the shell is produced anteroventrally is variable. Some specimens possess an anteroventral margin only slightly produced, whereas others demonstrate a markedly extended anteroventral margin.

Among those characters which demonstrate very little variation are the nature of the hinge and dentition, the nature of the lunule, and the nature of the anterior adductor impression and its junction with the pallial line.

REMARKS.–The species name honors Harold A. Rehder of the National Museum of Natural History, Smithsonian Institution.

Although the hinge plate of P. rehderi is not quite as broad as that of P. semperiana (Figures 3 and 4a), these two species demonstrate virtually identical hinge structure and dentition. Most other species of shell morphology correspond closely between the two species. P. semperiana differs from P. rehderi, however, in being larger, with a slightly shorter and more noticeably unsymmetrical lunule. The anterior dorsal area of P. semperiana tends to be slightly more distinct than that of P. rehderi. Similarly, the radial sculpture of the former species is more apparent but by no means dominant. Most aspects of the internal shell morphology of the two species are identical, including the nature of the anterior adductor impression and its junction with the pallial line.

Some criticism may be leveled at the description of a new species based on one lot from such a generalized locality as Brazil. Were it not for the fact that significant sculptural and other differences tend to separate P. rehderi and P. semperiana, I would be hesitant to describe the new form. However, there is definite morphological differentiation between the two. It is with some reluctance, therefore, that the new species is proposed; perhaps workers in the western South Atlantic region will be advised to search for additional records of this small species.

Parvilucina rehderi is certainly closely related to Parvilucina (Bellucina) semperiana (Issel, 1869). The latter is of the type of the subgenus Bellucina, hence P. rehderi is a member of the subgenus Bellucina. Thus, P. rehderi is the first species of Bellucina described from the Western Atlantic.

RANGE AND MATERIAL EXAMINED.–The species is known only by a single lot of material. The only locality record given is “Brasil.”

Genus Parvilucina

Subgenus Radiolucina, new subgenus

TYPE-SPECIES.–Phacoides amiantus Dall, 1901

DIAGNOSIS.–Shell suborbicular to slightly elongate, equivalve, moderately to well inflated, usually less than 20 mm. Anterior and posterior dorsal areas moderately to weakly indicated; lunule moderately elongate, narrow, unsymmetrical with portion in right valve slightly wider, not inserted beneath beaks, slightly to moderately depressed; escutcheon absent or very narrow. Dominant sculpture radial, consisting of less than 20 broad, prominently elevated ribs; concentric sculpture consisting of numerous fine lamellae, moderately spaced, elevated on and between ribs, definitely secondary in prominence to radials. Periostracum thin, obscure; ligament external, poorly depressed, extremely short. Hinge plate usually dorsoventrally broad; anterior and posterior lateral teeth well developed, single in right, paired in left valve; two cardinal teeth in left valve, right anterior cardinal tooth (3a) usually present but weak, right posterior cardinal tooth (3b) strong. Anterior adductor muscle impression relatively short, junction with pallial line usually ventral to midlength of muscle impression; anterior pedal retractor muscle impression separate from anterior adductor impression. Ventral margin crenulate.

REMARKS.–Since the original description of Phacoides amiantus Dall, 1901, the systematic position of this species has been subjected to several interpretations (Table 2). Most recently, Bretsky (1969) suggested the possibility that P. amiantus might represent a lineage distinct from any previously designated group. She rejected Chavan’s (1937, 1938) assignment of P. amiantus to Pleurolucina on grounds never adequately explained but presumably according to some of the points developed in the discussion below. Because she lacked specimens of the type-species of Bellucina (Lucina semperiana Issel, 1869) with which she could compare P. amiantus, Bretsky accepted Dall’s assignment of the latter to Bellucina, noting that P. amiantus and L. semperiana could be of distinct lineages.

I have had an opportunity to compare Phacoides amiantus with specimens of Lucina semperiana and several other species (Figure 4, Table 3). I believe P. amiantus represents a distinct lineage within the Lucinidae, for reasons given in the discussion. Thus, I propose the new subgenus, Radiolucina, for those species similar to Phacoides amiantus Dall, 1901. This includes the Recent Eastern Pacific species Lucina cancellaris Philippi, 1846, and several Tertiary fossils, including Phacoides waccamawensis Dall, 1903, Phacoides tuomeyi Dall, 1903, Lucina katherinepalmerae Weisbord, 1964, Phacoides nereidedita Maury, 1910, and Phacoides euphaea Gardner, 1926.

DISCUSSION.–The discussion is primarily concerned with two considerations previously noted by Bretsky (1969): (1) the possibility that Parvilucina, Bellucina (fide Bretsky; that is, those species of Radiolucina herein), and Pleurolucina form “a phyletic progression with sequential decrease in number and increase in size of radial ribs” (Bretsky, 1969:72); and (2) the possibility that “the type of Bellucina is actually a Parvilucina [or otherwise not related to Lucina amiantus, JCB] in which case a new subgeneric name would have to be proposed for the distinctive group of species to which L. amiantus belongs” (Bretsky, 1969:278).

RADIAL ORNAMENTATION AND THE CONDITION IN PLEUROLUCINA.–Both of the above considerations involve, to a greater or lesser degree, the nature of radial ornamentation on Pleurolucina. It is generally agreed among the major revisers of the Lucinidae that Pleurolucina possesses radial “ribs.” An undefined assumption seems to hold that these ribs are primary elevations upon the shell exterior and arise in a manner analogous with the ribs of certain species of Parvilucina and Radiolucina. By comparing the radial ornamentation of several species of each of these groups, certain trends or central tendencies within and between the groups become apparent. Phylogenetic interpretations may vary according to the direction one assigns to these tendencies. For example, Bretsky (1969) presents evidence which favors recognition of a Parvilucina-Radiolucina lineage (Figure 5). I concur with Bretsky’s hypothesis, considering the lineage as possessing “true” radial ribs (i.e., ribs formed as primary localized elevations above an external shell base) which demonstrate the tendency to become more prominently elevated and fewer in number. Bretsky also suggests that Pleurolucina, with a few broad radial “ribs,” could form a terminal element of the Parvilucina-Radiolucina lineage. I disagree.

The radial ribs of Pleurolucina do not seem to be analogous to those of the Parvilucina-Radiolucina complex. The ribs of the former apparently are not primary elevations above the shell base; rather they are secondary “elevations” formed subsequent to and mostly as a result of the formation of radial sulci (i.e., localized depressions below the shell base).

FIGURE 4.–Comparison of the type-species of the subgenera Bellucina, Radiolucina, and Pleurolucina: a, Parvilucina (Bellucina) semperiana (Issel, 1869), USNM 598440, right valve exterior, length 3.2 mm; b, Parvilucina (Radiolucina) amiantus (Dall, 1901), USNM 64276, right valve exterior, length 8.6 mm; c, Lucina (Pleurolucina) leucocyma Dall, 1886, USNM 83140, right valve exterior, length 6.8 mm.

Consider the condition of L. (Pleurolucina) hendersoni, new species. The crest or high point of each of the “ribs” of L. hendersoni is equivalent to the base level of elevation on the shell exterior. Concentric lamellae are elevated above this base level, while several shell sulci or grooves are incised below it. [According to a figure (Keen, 1958: 97), L. leucocymoides (Lowe, 1935) of the eastern Pacific has radial elements of sculpture almost identical to that of L. hendersoni. I have not seen specimens of L. leucocymoides.] When sulci fail to be impressed on the ventral third of the valve, the base level is essentially at the same relative elevation as the “rib” crests on the more dorsal portion of the valve. The condition of L. hendersoni is in contrast to that of certain Radiolucina which also demonstrate a slight tendency toward ventral obsolescence of radial sculpture [e.g., Parvilucina (Radiolucina) waccamawensis (Dall, 1903) of the South Carolina Pliocene, or P. (R.) tuomeyi (Dall, 1903), a Miocene species from the same geographic region]. In these species the base level on the central valve exterior corresponds not with the crests of the ribs, but with the floor of the grooves between the ribs as they exist on the more dorsal portions of the valve exterior.

FIGURE 5.–Possible sequence of a Parvilucina–Radiolucina lineage with Recent species as examples. All specimens right valves with external view: a, Parvilucina (Parvilucina) tenuisculpta (Carpenter, 1864), USNM 108826, length 13.6 mm; b, P. (P.) multilineata (Tuomey and Holmes, 1856), USNM 60981, length 5.1 mm; c, P. (P.) pectinella (C. B. Adams, 1852), MCZ 155597, length 6.8 mm; d, P. (P.) costata (d’Orbigny, 1842), USNM 683297, length 9.4 mm; e, P. (Radiolucina) amiantus (Dall, 1901), USNM 64276, length 8.6 mm.

An alternative to Bretsky’s (1969) Parvilucina, sensu lato, Pleurolucina progression hypothesis takes into consideration the nature of radial ornamentation in L. hendersoni. According to the new hypothesis Pleurolucina is visualized acquiring radial “ribs” secondarily after sulci formation rather than as primary formations elevated above a shell base. The radial ornamentation of L. leucocyma could have arisen just as easily by this secondary, sulci-related process.

The first stage of the progression might be similar to the condition in L. sombrerensis (Dall, 1901), a Recent western Atlantic species. The shell of this species almost lacks radial ornamentation. On the anterior slope of each valve, however, are two radial sulci. The anteriormost sulcus is slightly weaker than the other but extends entirely to the anterior margin. The second or more posteriorly located sulcus fails to extend entirely to the anteroventral margin. Figure 6a illustrates the external ornamentation on the shell of L. sombrerensis.

The next stage of the series is represented by a form like L. hendersoni, or the fossil L. follis Olsson, 1964, and has been described (Figure 6c). The dominance of sulci over “ribs” is obvious in this stage.

L. leucocyma (Figure 6d) is an example of the final stage where the radial “ribs” take the form of definite elevations. This seems to have been achieved by two simultaneous developments: (1) the sulci deepen and widen, while (2) the shell undergoes a moderate anteroposterior compression (i.e., a dorsoventral elongation). These two processes acting together cause slight folding of the radial ornamentation, additional deepening of the sulci, and elevation of the “ribs.”

If Pleurolucina originated in a manner such as has just been postulated, perhaps additional evidence could be presented suggesting a closer relationship between it and some other lucinid group lacking primary radial elevations of the shell exterior (i.e., lacking ribs). Such is the case between Pleurolucina and Lucina, sensu stricto.

THE RELATIONSHIP BETWEEN LUCINA, SENSU STRICTO, AND PLEUROLUCINA.–A cursory examination of the type-species of Lucina, sensu stricto, and Pleurolucina gives one cause to doubt that these species are closely related. Not only is L. leucocyma considerably smaller than L. pensylvanica and, unlike the latter, possesses “radial” ornamentation, but the concentric ornamentation of the valve exterior appears to be different. L. leucocyma possesses relatively broad concentric lamellae separated by narrow interspaces, whereas L. pensylvanica has fewer narrow primary concentrics separated by wider interspaces. Closer examination of the shell morphology of the two groups reveals, however, several important similarities between them.

The similarity of hinge structures has already been mentioned. This similarity is an inconclusive correspondence shared by a number of lucinid groups. Other common features of Lucina, sensu stricto, Pleurolucina, and Here (a group very closely related to Lucina, sensu stricto, but differing from it primarily by the nature of the lunule) can be listed and compared with corresponding features of the Parvilucina-Radiolucina complex.

1. The lunule of Pleurolucina is either very small and inserted below and posterior to the beaks as in Lucina, sensu stricto, or deeply incised in a manner similar to the condition in Here; whereas the lunule of Parvilucina, sensu lato, tends to be more elongate.

2. Most species of Pleurolucina possess a relatively broad anterior dorsal area or pseudolunule as in Lucina, sensu stricto, and Here; whereas the anterior dorsal area in Parvilucina, sensu lato, usually is considerably narrower and less noticeable.

3. The junction of the pallial line with the anterior adductor muscle impression in Pleurolucina is located dorsal to midlength of the muscle scar as in the genus Lucina; whereas this condition is observed rarely among species of Parvilucina, sensu lato.

4. The posterior sulcus of Pleurolucina is usually well impressed and the posteroventral margin is noticeably incised at the termination of this sulcus; this condition occurs in Lucina, sensu stricto, and Here, but is expressed weakly or not at all in Parvilucina, sensu lato.

FIGURE 6.–Possible derivation of radial ornamentation in Pleurolucina: a, Lucina (Pleurolucina) sombrerensis (Dall, 1901), USNM 329253, length 5.9 mm, left valve exterior; b, line drawing of preceding, indicating position of radial ornamentation; c, L. (P.) hendersoni, new species, USNM 503399, length 12.3 mm, left valve exterior; d, L. (P.) leucocyma Dall, 1886, USNM 83140, length 6.8 mm, right valve exterior photographically reversed to facilitate comparison with other specimens.

The major weakness in the case for a close phylogenetic relationship between Pleurolucina and Lucina, sensu stricto, has been the nature of external ornamentation of the two groups. Similarly, this has seemed to be one of the major arguments for a relationship between Pleurolucina and Parvilucina, sensu lato. If differences of shell ornamentation between the former pair could be satisfactorily accounted for, the case for a closer relationship between these groups would be considerably strengthened. By resolving the matter of radial ornamentation in Pleurolucina in the preceding section, there remains only to demonstrate some degree of relationship between the two groups with regard to concentric lamellae. L. hendersoni, new species, can be employed again to represent an intermediate stage between the two types.

There should be little doubt that L. hendersoni is a Pleurolucina, considering the typical five sulci of the shell exterior and the close correspondence of its internal shell morphology with that of L. leucocyma. Its concentric lamellae are quite unlike those of L. leucocyma, being very narrow, well elevated, and broadly spaced in a manner similar to that expressed by L. pensylvanica. Thus, species such as L. hendersoni seem to suggest a transitional stage betwene Lucina, sensu stricto, and Pleurolucina with respect to concentric ornamentation and the other morphological shell characters mentioned above.

A good case can be made, then, for a close phylogenetic relationship between Pleurolucina and Lucina, sensu stricto. Now consider the relationships of Pleurolucina and Parvilucina, sensu lato.

RELATIONSHIPS OF RADIOLUCINA, BELLUCINA, AND PLEUROLUCINA.–Radiolucina, Bellucina, and Pleurolucina are compared primarily to demonstrate the need for the new subgeneric designation, and also to establish a basis for additional phylogenetic considerations (Figure 4). Table 3 summarizes certain morphological shell characteristics of each of the type-species. Most species of Parvilucina, sensu stricto, produce weak or relatively feeble elements of radial ornamentation; hence, this subgenus is omitted from the present discussion. Bretsky (1969) has pointed out the tendency of some Parvilucina, sensu stricto, species to produce relatively stronger radial elements which approach the condition of Radiolucina.

Radiolucina and Bellucina share a number of very similar morphological shell characters including several not included in Table 3. Hence, both are considered of the Parvilucina, sensu lato, lineage (see Bretsky, 1969). The justification for designating the former group as a new subgenus, however, is based primarily on differences with respect to (1) the nature of the ventral margin, (2) certain aspects of cardinal and lateral dentition, and especially (3) the nature of external ornamentation. The latter difference is particularly pronounced. Radial ribs of Radiolucina are unquestionably dominant over concentric lamellae, usually tending to obscure the latter. In Bellucina the condition is reversed with the concentric lamellae being dominant. Radial sculpture in Bellucina is restricted to interspaces between concentrics giving the appearance of numerous separate, short columnar elements.

The basic difference between Pleurolucina and Radiolucina with respect to radial ornamentation has already been discussed. In addition, there are a number of other dissimilarities between analgous characters of the two groups. These include (1) the nature of the lunule, (2) the position of the junction of the pallial line with the anterior adductor impression, (3) the nature of the anterior dorsal area, and to a lesser degree (4) the nature of the ventral margin and (5) certain aspects of hinge dentition (Table 3). Similar differences exist between Pleurolucina and Bellucina. Thus, Pleurolucina appears to be more remotely related to Radiolucina and Bellucina than the latter two are related to one another. This is an additional justification, then, for considering Pleurolucina outside the Parvilucina, sensu lato, lineage.

CONCLUSIONS.–A systematic revision of those groups listed in Table 1 is proposed. A summary of the revision follows, including names of only those species given primary consideration herein.

Genus Lucina

     Subgenus Lucina, sensu stricto, including L. pensylvanica.

     Subgenus Here, including L. richtofeni.

     Subgenus Pleurolucina, including L. leucocyma, L. hendersoni, and L. sombrerensis.

     Genus Parvilucina

     Subgenus Parvilucina, sensu stricto, including P. tenuisculpta.

     Subgenus Radiolucina, including P. amiantus.

     Subgenus Bellucina, including P. semperiana and P. rehderi.

TABLE 3.–Comparison of certain morphological shell characters for three lucinid subgenera

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Index

(Names of new subgenus, species, and page numbers of principal accounts in italics).

amabilis, Lucina, 2

amiantus, Bellucina, 3

Linga, 3

Lucina, 3, 10

Parvilucina, 11 (fig.), 12 (fig.), 15

Phacoides, 2, 3, 4. 9, 10

Pleurolucina, 3

Anodontia, 1

Bellucina, 1, 2, 3, 4, 9, 10, 11, 15, 16

amiantus, 3

semperiana, 3

cancellaris, Lucina, 10

carnifera, Lucina, 2

Cavilinga, 2, 3

trisulcata, 3

Cavilucina, 3

trisulcata, 3

Codakia, 1

columbella, Linga, 3

Lucina, 2

costata, Parvilucina, 12 (fig.)

Dallucina, 2

Divaricella, 1

endentula, Parvilucina, 2

euphaea, Phacoides, 10

excavata, Lucina, 2

follis, Lucina, 2, 7, 13

Here, 2, 3, 13, 14, 15

richtofeni, 3

hendersoni, Lucina (Pleurolucina) 4 (fig.), 5 (fig.), 11, 13, 14 (fig.), 15

katherinepalmerae, Lucina, 10

leucocyma, Linga, 3

Lucina, 2, 3, 7, 11 (fig.), 13, 14 (fig.), 15

Phacoides, 2, 3

Pleurolucina, 3

leucocymoides, Lucina, 11

Linga, 2, 3

amiantus, 3

columbella, 3

pensylvanica, 3

richtofeni, 3

semperiana, 3

trisulcata, 3

Loripes, 1

Lucina, 1, 2, 3, 4, 7, 13, 14, 15

amabilis, 2

amiantus, 3, 10

cancellaris, 10

carnifera, 2

columbella, 2

excavata, 2

follis, 2, 7, 13

(Pleurolucina) hendersoni, 4 (fig.), 5 (fig.), 11, 13, 14 (fig.), 15

katherinepalmerae, 10

leucocyma, 2, 3, 7, 11 (fig.), 13, 14 (fig.), 15

leucocymoides, 11

pensylvanica, 3, 7, 13, 15

richtofeni, 2, 3, 15

sombrerensis, 13, 14, 15

tenuisculpta, 2, 3

trisulcata, 2, 3

Miltha, 1

multilineata, Parvilucina, 12 (fig.)

Myrtea, 1

nereidedita, Phacoides, 10

Parvilucina, 2, 3, 10, 13, 14, 15

amiantus, 11 (fig.) 12 (fig.), 15

costata, 12 (fig.)

edentula, 2

miltilineata, 12 (fig.)

pectinella, 12 (fig.)

(Bellucina) rehderi, 7 (fig.), 8, 9, 15

semperiana, 11 (fig.), 15

tenuisculpta, 3, 12 (fig.), 15

trisulcata, 3

tuomeyi, 13

wacamawensis, 13

Paslucina, 2

pectinata, Tellina, 2

pectinella, Parvilucina, 12 (fig.)

pensylvanica, Linga, 3

Lucina, 3, 7, 13, 15

Phacoides, 2, 3

Venus, 2

Phacoides, 1, 2, 3

amiantus, 2, 3, 4, 9, 10

euphaea, 10

leucocyma, 2, 3

nereidedita, 10

pensylvanica, 2, 3

richtofeni, 2, 3

semperiana, 2, 3

tenuisculpta, 2, 3

trisulcata, 2, 3

tuomeyi, 10

wacamawensis, 10

Pleurolucina, 2, 3, 7, 10, 11, 13, 14 (fig.), 15, 16

amiantus, 3

leucocyma, 3

Quasilucina, 2

(Radiolucina), 2, 9, 10, 11, 15, 16

rehderi, Parvilucina (Bellucina), 7 (fig.), 15

richtofeni, Here, 3

Linga, 3

Lucina, 2, 3, 15

Phacoides, 2, 3

semperiana, Bellucina, 3

Linga, 3

Parvilucina, 11 (fig.), 15

Phacoides, 2, 3

sombrerensis, Lucina, 13, 14, 15

Tellina, 2

pectinata, 2

tenuisculpta, Lucina, 2, 3

Parvilucina, 3, 12 (fig.), 15

Phacoides, 2, 3

trisulcata, Cavilinga, 3

Cavilucina, 3

Linga, 3

Lucina, 2, 3

Parvilucina, 3

Phacoides, 2, 3

tuomeyi, Phacoides, 10

Parvilucina, 13

Venus, 2, 3

pensylvanica, 2

wacamawensis, Parvilucina, 13

Phacoides, 10

__________

¹Chavan (1937) and several other authors have cited Spengler, 1784, as the author of L. jamaicensis. Chemnitz (1784) described Venus jamaicensis from the cabinet of Spengler. The specimens Chemnitz described were apparently labeled “jamaicensis” in Spenglcr’s hand for purposes of identification within the collection (“Diese muschel wird im Spenglerischen Cabinette Venus Jamaicensis genannt, welche Benennung ich sehr gerne behbehalte,” Chemnitz, 1784). Spengler did not publish the name jamaicensis prior to publication of Tellina pectinala Gmelin, 1791. Thus, Chemnitz (1784), not Spengler, should be credited with the first use of jamaicensis in a published work. Note that Chemnitz (1784) is on the list of officially rejected works of the International Commission on Zoological Nomenclature.

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Simple tabulations in the text (e.g., columns of data) may carry headings or not, but they should not contain rules. Formal tables must be submitted as pages separate from the text, and each table, no matter how large, should be pasted up as a single sheet of copy.

Use the metric system instead of, or in addition to, the English system.

Illustrations (line drawings, maps, photographs, shaded drawings) can be intermixed throughout the printed text. They will be termed Figures and should be numbered consecutively; however, if a group of figures is treated as a single figure, the components should be indicated by lowercase italic letters on the illustration, in the legend, and in text references: “Figure 9b.” If illustrations (usually tone photographs) are printed separately from the text as full pages on a different stock of paper, they will be termed Plates, and individual components should be lettered (Plate 9b) but may be numbered (Plate 9: figure 2). Never combine the numbering system of text illustrations with that of plate illustrations. Submit all legends on pages separate from the text and not attached to the artwork. An instruction booklet for the preparation of illustrations is available from the Press on request.

In the bibliography (usually called “Literature Cited”), spell out book, journal, and article titles, using initial caps with all words except minor terms such as “and, of, the.” For capitalization of titles in foreign languages, follow the national practice of each language. Underscore (for italics) book and journal titles. Use the colon-parentheses system for volume, number, and page citations: “10(2):5-9.” Spell out such words as “figures,” “plates,” “pages.”

For free copies of his own paper, a Smithsonian author should indicate his requirements on “Form 36” (submitted to the Press with the manuscript). A non-Smithsonian author will receive 50 free copies: order forms for quantities above this amount with instructions for payment will be supplied when page proof is forwarded.