Parvamussium biformatum Dijkstra & Maestrati 2008, n. sp.

Figs 5-10

TYPE MATERIAL. — Holotype (spm) MNHN 20448.

TYPE LOCALITY. — Solomon Islands, 08°20’S, 160°39’E, 453-542 m [SALOMON 1: stn DW 1770].

MATERIAL EXAMINED. — Only known from the type material.

DISTRIBUTION. — Only known from the type locality.

DESCRIPTION. — Shell small, fragile, circular, semi-transparent, inequivalve, rather inflated, left valve slightly more convex than right valve, auricles unequal in shape and size, umbonal angle about 110˚, colour whitish with brownish maculations.

Left valve with reticulated sculpture produced by intersecting commarginal and radial sculpture in early to medium growth stages, and with more prominent commarginal sculpture in late growth stages. Commarginal lamellae commence at 0.5 mm shell height and extend to the ventral margin, more closely spaced near the margin. Irregularly and widely spaced, intercalated radial riblets commence at 1 mm shell height and also extend to the ventral margin, weaker and more closely spaced near the margin. Anterior and posterior auricles with closely spaced commarginal lamellae, umbonal area smooth.

Right valve with regularly arranged commarginal lirae (1 per mm on central part of disc). Anterior auricle with closely spaced, coarse commarginal lamellae, more delicate on posterior. Hinge line straight. Byssal notch shallow, byssal fasciole lacking.

Prominent internal ribs, 8 plus 2 rudimentary and 1 weak auricular on each side, commence at 1 mm shell height and extend close to ventral margin.

Dimensions of holotype: H 4.6, W 4.9, D 1.6.

REMARKS. — Parvamussium biformatum resembles P. multiliratum Dijkstra, 1995 in shape, but differs strongly in sculpture (reticulate in early ontogeny and commarginal in late ontogeny; P. multiliratum commarginal throughout) and internal ribs (8 prominent + 2 rudimentary, P. multiliratum 14 delicate + 2 rudimentary). Moreover, P. biformatum has a dull umbonal area, that of P. multiliratum being glossy, and it also differs in colour (P. biformatum is opaque whitish and brownish, P. multiliratum more whitish transparent).

Parvamussium biformatum somewhat resembles Parvamussium retiaculum Dijkstra, 1995 in sculpture. The two species have a similar reticulate sculpture on the central part of the disc, but differ strongly ventrally (P. biformatum commarginal, P. retiaculum reticulate). Moreover, P. biformatum is circular in shape with a smaller anterior auricle, P. retiaculum being more oblong. Parvamussium biformatum has fewer internal ribs (8 + 2 rudimentary, P. retiaculum 12) and they commence earlier. The two species are semi-transparent, but they differ in colour (P. biformatum whitish with brownish maculations, P. retiaculum whitish).

ETYMOLOGY. — Two different sculptures on the left valve (Latin biformatus, adjective = two forms).