Allogramma formosa, (JEFFREYS, 1882)

ALLOGRAMMA FORMOSA (JEFFREYS, 1882) View in CoL

( FIGS 6, 7)

Description

Shell: Subquadrate,inflated,equivalveandinaequilateral; extremely fragile, pearl lustre; outer sculpture well marked with two long radial ridges from the umbo until the posterior ventral margin of the valves, small spines in the posterodorsal portion of the shell; with a lithodesma.

Mantle: Mantle margin with two fused points, anteriorly forming a wide pedal gape and posteriorly forming the siphons; posteroventrally, mantle margin fusion formed by inner and middle folds (Type B) ( Yonge, 1982); absence of a fourth pallial aperture.

Siphons: Separated, different in size and outline; inhalant siphon, large, modified in a raptorial appendage, usually contracted into the pallial cavity; exhalant siphon short, tube-shaped; both surrounded at the base by a ring of ~50 smaller tentacles closest to the aperture and by ~60 bigger tentacles peripherally located.

Ctenidia: Eulamellibranch, very reduced, non-plicate and horizontally aligned; complete, with two demibranchs.

Labial palps: Non-lamellate, extremely complex, outer and inner palps medially fused forming two pouches or buccal sacs, one small anterior and another large posterior; unfused tips forming narrow fluted funnels laterally.

Musculature: Posterior and anterior adductor muscles present and isomyarian; with posterior and anterior pedal retractor muscle; absence of taenioid muscle.