Sub-order V.—Cardiacea.
Two pallial sutures. Siphons generally short. Foot cylindrical, more or less elongated, byssogenous. Gills much folded. Shell equivalve, with radiating costae and external ligament.
Fam. 1. Cardiidae.—Mantle slightly closed; siphons very short, surrounded by papillae which often bear eyes; foot very long, geniculated; pallial line without sinus; two adductors, Cardium; British. Pseudo-kellya. Byssocardium; Eocene. Lithocardium; Eocene.
Fam. 2. Limnocardiidae.—Siphons very long, united throughout; shell gaping; two adductors; brackish waters. Limnocardium; Caspian Sea and fossil from the Tertiary. Archicardium; Tertiary.
Fam. 3. Tridacnidae.—Mantle closed to a considerable extent; apertures distant from each other; no siphons; a single adductor; shell thick. Tridacna. Hippopus.
Sub-order VI.—Chamacea.
Asymmetrical, inequivalve, fixed, with extensive pallial sutures; no siphons. Two adductors. Foot reduced and without byssus. Shell thick, without pallial sinus.
Fam. 1. Chamidae.—Shell with sub-equal valves and prominent umbones more or less spirally coiled; ligament external. Chama. Diceras; Jurassic. Requienia; Cretaceous. Matheronia; Cretaceous.
Fam. 2. Caprinidae.—Shell inequivalve; fixed valve spiral or conical; free valve coiled or spiral; Cretaceous. Caprina. Caprotina. Caprinula, &c.
Fam. 3. Monopleuridae.—Shell very inequivalve; fixed valve conical or spiral; free valve operculiform; Cretaceous. Monopleuron. Baylea. The two following families, together known as Rudistae, are closely allied to the preceding; they are extinct marine forms from Secondary deposits. They were fixed by the conical elongated right valve; the free left valve is not spiral, and is furnished with prominent apophyses to which the adductors were attached.