« Shell equivalve, generally transverse, more or less compressed, or triangular and inequilateral, with posterior side almost always shorter than the anterior and sharply truncated, or else oval and almost equilateral; test thick, porcellaneous; external surface generally smooth, sometimes decorated with concentric ridges, but never having longitudinal ribs; epidermis fairly solid, shiny, horny. Peaks slightly salient and opisthogyrous. Ligament external, poorly developed in general and communicating, by a strong notch at each valve, with an internal ligament, the cartilage or resilium, which is housed in a dimple in the shape of a deep spoon, the chondrophore, more or less protruding in the inside of the valve. » – E. Lamy: “Révision des Mesodesmatidae vivants du Muséum National d’Histoire Naturelle de Paris”, Journal de Conchyliologie vol.62, Paris 1914, p2.
 

Lamarck introduced the name, but this is Philippi who gave the very first acurate description of this genus, coupled with the taxon Donacilla lamarckii (junior synonym of Mactra cornea Poli, 1791). « Shell transverse, equivalve, inequilateral, short posteriorly. Cardinal teeth strong, divaricate, two on the right valve and three or four on the left valve, the exterior ones (usually the laterals) elongated. Ligament internal. Pallial sinus remote, rather deep. » – R. A. Philippi: Enumeratio molluscorum Siciliae vol.I, Berlin 1836, p.37.