de Férussac, 1822
Shell equivalve, inequilateral, beaks in the front half, sometimes at or close to the anterior end. Triangular, rhomboidal or oval in outline. Periostracum prominent, often thick and spiny. Ligament internal or external. Hinge line without teeth but sometimes with tooth-like crenulations. In Crenella these crenulations are gathered onto a process under the beaks which is continuous anteriorly with the crenulate shell margin; this condition has been called pseudo-taxodont. Adductor muscle scars are normally markedly unequal, the anterior being much the smaller. Exceptionally, as in Idas , the muscle scars are almost equal. No pallial sinus.
Source: Tebble, 1966.
The following subtaxa of this order occur in the region: