Family Lyonsiidae

Shell size to 15 mm; shell elongate, inequivalve, with posterior region prolonged and narrow. Postero-dorsal margin concave. Umbo sub-central. Sculpture of fine, wide separated concentric lines and radial ridges. Periostracum thin, present mostly as expansions of lines and ridges. The young attach by delicate byssus to broken shells or small pebbles. Lives in the mud or sandy-mud back bay substrates.

Shell size to 25 mm; shell elongate, inequivalve, irregularly shaped. Posterior region usually wider than anterior. Postero-dorsal margin concave. Umbo anterior. Shell nacreous, translucent-brownish. Found in sponges or attached by thin byssal thread to other shells.