Shell equivalve, small to large, often gaping posteriorly. « The most conspicuous morphological character that defines the mactrids is the inverted V-shaped cardinal tooth in the left valve, formed by two single cardinal teeth. In addition, the right valve’s cardinal teeth are not fused and the anterior and posterior lateral teeth have, in general, only one cusp. » – Signorelli & Pastorino: “A Revision of the Living Mactridae (Bivalvia: Autobranchia) from Northern Argentina and Uruguay”, American Malacological Bulletin vol.30(1), p.85.
 

« Shell oblong, rather ventricose, thick, equilateral, radiately ribbed, hinder slope rugose, hinder gape small; cardinal teeth of left valve compressed, nicked; anterior lateral tooth nearly perpendicular. [../Ligament subexternal, marginal, not separated from the cartilage.] » – J. E. Gray: “A revision of the genera of some of the families of Conchifera or bivalve shells”, Annals and Magazine of Natural History ser.2 v.XI, London 1853, p.42.
 

« Shell transverse, inequilateral, gaping at the ends; two oblique and divergent cardinal teeth, accompanying a large dimple for the ligament; lateral teeth null, or contiguous to the dimple. » – J. B. M. de Lamarck: “Prodrome d’une nouvelle classification des coquilles”, Mémoires de la Société d’Histoire Naturelle de Paris vol.I, Paris 1799, p.85.
 

Shell medium-sized to large, thin, equivalve, nearly equilateral, subtriangular, wider than high, slightly gaping anteriorly; pallial margin rounded; beaks small, prosogyrate. Hinge provided with two long lateral teeth, one per side, which lock within two laminae of the opposite valve.
 

Shell medium-sized to large, thick, equivalve, nearly equilateral, subtriangular, often with a larger ratio height/width and a less rounded pallial margin than in Mactra, and with subangular extremities; the valves do not gape; pallial margin rounded; « ligament sub-external, marginal, not separate from the cartilage » – G. B. Sowerby: A conchological manual third edition, London 1846, p.264.