Pleurolucina hendersoni (Britton 1972)

Lucina (Pleurolucina) hendersoni

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.

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).