Truncilla donaciformis shell description:

"Shell rather small, subsolid, irregularly ovate, subinflated, inequilateral, with moderately high and full, though slightly flattened, beaks, whose sculpture consists of fine, doubly-looped ridges, the hinder loop being quite irregular on the sharp posterior ridge; surface with irregular growth lines, sometimes slightly plicate or corrugated on the posterior slope, generally shining, pale or yellowish-green with a beautiful pattern of darker green rays. These rays are sometimes entire, but are generally broken up into arrow-head or zigzag markings; left valve with two compressed pseudocardinals and two laterals; right valve with one pseudocardinal and one lateral; beak cavities shallow; muscle scars impressed, the posterior ones round; nacre bluish-white.  The female shell is apparently always smaller than that of the male and has a decided marsupial swelling, the sharp posterior point being a little more elevated than that of the male shell” (Simpson, 1914; Mather, 2007).