Parvamussium orbiculatum, Dijkstra & Maestrati, 2013

Dijkstra, Henk H. & Maestrati, Philippe, 2013, New species and new records of bathyal living Pectinoidea (Bivalvia: Propeamussiidae: Pectinidae) from the Southwest Pacific, Zoosystema 35 (4), pp. 469-478 : 472

Parvamussium orbiculatum n. sp. ( Fig. 1 A-H)

TYPE MATERIAL. — Solomon Islands. SALOMON 2, stn CP 2176, 9°09’S, 158°59’E, 600-875 m, holotype spm ( MNHN 25749). — Same data, paratype spm ( MNHN 25750).

TYPE LOCALITY. — Solomon Islands, SW Russell Island, 9°09’S, 158°59’E, 600-875 m (SALOMON, stn CP 2176).

ETYMOLOGY. — Shell disc circular (Latin orbiculatus, adjective meaning circular).

OTHER MATERIAL EXAMINED. — Coral Sea. EBISCO, stn DW 2543, 21°10’S, 158°38’E, 670-745 m, 1 spm.

DESCRIPTION

Shell up to c. 5 mm high, fragile, circular, inequivalve, hyaline, whitish, left valve slightly more convex than right valve, auricles subequal, umbonal angle about 110°.

Both valves with 11 short internal ribs, commencing in late ontogeny, an auricular riblet on posterior auricle. External sculpture clearly visible from interior.

Left valve disc and auricles smooth and glossy with some weak commarginal growth lines.

Right valve with evenly spaced commarginal lirae. Auricles with delicate and weak commarginal lamellae, more prominent on anterior auricle and more closely spaced on posterior. Radial ridge along byssal fasciole. Hinge line straight, with a few scales near dorsal margin, more prominent on anterior auricle. Byssal notch shallow, byssal fasciole small.

Dimension of holotype: H 5.0 mm, W 5.0 mm, D 1.4 mm.

REMARKS

The present species is morphologically close to Parvamussium multiliratum Dijkstra, 1995 , known from the lower bathyal and upper abyssal southwestern Pacific, but differs in size ( P. orbiculatum n. sp. 5 mm, P. multiliratum 8 mm in height), in sculpture ( P. orbiculatum n. sp. left valve smooth, P. multiliratum with commarginal lamellae), in internal ribbing ( P. orbiculatum n. sp. 11 short ribs, P. multiliratum 14 long ribs with 2 rudimentary).

Figures

FIG. 1. — Parvamussium orbiculatum n. sp.: A, B, E, F, holotype MNHN 25749;A, external view of the left valve;B, external view of the right valve; E, detail of the commarginal striae of the external side of the right valve; F, idem, detail inter striae; C, D, G, H, paratype MNHN 25750;C, internal view of the left valve; D, internal view of the right valve;G, Prodissoconch of the right valve;H, prodissoconch of the left valve. Scale bars: A-D, 1 mm; E, 100 µm; F, 20 µm; G, H, 50 µm.