Lasmigona complanata shell description:

 “Shell large, solid, irregularly elliptical, obovate or subrhomboid, inequilateral, compressed; beaks low, decidedly flattened, their sculpture consisting of coarse, doubly-looped, subnodulous ridges with radiating raised threads behind; post-dorsal part of the young shell produced into a high, often angular wing, which is truncated behind making the outline at this period of growth somewhat triangular; as the shell becomes old this wing breaks off and is worn away; posterior ridge low, sometimes double and ending behind in a biangulation on the median line; base rounded, full behind the middle; surface with rude growth lines, often somewhat plicate on the posterior slope; epidermis dark green in young shells, blackened in old ones, scarcely shining; left valve with two strong, uneven pseudocardinals, the hinder generally divided sometimes somewhat  ^-shaped;  right valve with one or two pseudocardinals, which are frequently split up and imperfect; laterals very much blurred in both valves; muscle scars large; nacre white, thickened in front, with a wide border outside the pallial line” (Simpson 1914, Mather 2007).