Arcoida

The Arcoida is an extant order of bivalve molluscs. This order dates back to the lower Ordovician period. They are distinguished from related groups, such as the mussels, by having a straight hinge to the shells, and the adductor muscles being of equal size. The duplivincular ligament, taxodont dentition, and a shell microstructure consisting of the outer crossed lamellar and inner complex crossed lamellar layers are defining characters of this order.

Seven families are currently recognised within the order, including the well-known ark clams or ark shells in the family Arcidae.

Taxonomy of the Arcoida

The order Acoida, as the suborder Arcacea, is included in the order Taxodonta by R.C. Moore, 1952, chacterised by simple hinge-line dentition consisting of small, numerous, similar hinge teeth, separate mantle lobes, poorly developed siphons, and filibranch gills.

In 2010, Bieler, Carter & Coan proposed a new classification system for the Bivalvia which combines the taxodont Arcoida with the dysodont Limnoida Mytiloida, Ostreoida and Pteroida as the Pteriomorphia. Subtaxa included in the Arcoida are shown below.