« Shell equivalve, laterally compressed to subcylindrical in cross-section, with a narrowly elongate, often sword-like or razor-like shape, very inequilateral and gaping at both ends. Umbones not prominent, frequently near the anterior end of dorsal margin. Outside of shell essentially with concentric growth marks, often changing abruptly in direction along a diagonal line running from the umbones to the posteroventral end of valves. […] Ligament external, set in an elongate linear groove of posterodorsal margin. Hinge feeble, with at least one peg-like cardinal tooth in each valve, and sometimes one or two lamellar posterior teeth » (fao.org).
 

« Shell bivalve, equivalve, transversely elongated, gaping at both ends; with very small umbos, not protruding. Cardinal teeth small, in variable number, sometimes zero, rarely divergent, more rarely fitting into dfossulas; ligament external. » – J.-B. M. de Lamarck: Histoire naturelle des animaux sans vertèbres vol.V, Paris 1818, p.449.