Family Myidae

Shell size to 10 mm; shell thin, fragile, ovate-elongate. Right valve may be more inflated. Posterior end truncated, gaping, sometimes twisted to left or right. Periostracum may prolong posterior end of shell (see figures). External surface with fine commarginal ribs. Right valve with chondrophore under umbo. Color translucent- to waxy-white. This species is a nestler, settling in pre-existing cavities on rocks, corals, or other shells. Sphenia antillensis Dall & Simpson, 1901, is a synonym of this species.