Order IV. Eulamellibranchiata
Mantle edges united at one or more points, branchiae with interfilamentary junctions which are always vascular, genital glands not opening into the kidneys, usually two adductor muscles.
Sub-order I. Submytilacea.—Mantle edges more or less open, anal orifice distinct, usually no siphons, pallial line usually simple, cardinal and lateral teeth well marked.
Fam. 1. Carditidae.—Foot with a byssus or groove, branchiae large, unequal; shell equivalve, solid, radiately grooved, one or two oblique cardinal teeth, one or two laterals. Silurian ——. Principal genera: Venericardia, Cardita, Carditella, Carditopsis, Milneria; Pleurophorus (Palaeozoic), Anodontopsis (Silurian).
Fam. 2. Astartidae.—A short anal siphon, labial palps large; shell triangular, thick, ligament external, hinge with two or three cardinals in each valve, laterals obscure.? Devonian ——. Principal genera: Astarte; Pachytypus (Jurassic), Plesiastarte (Eocene), Parastarte, Woodia, Opis (Secondary strata), Prosocoelus (Devonian).
Fam. 3. Crassatellidae.—Mantle with anal orifice or open; shell equivalve, thick, subtriangular, ligament in an internal fossette, hinge with two cardinals, laterals produced. Cretaceous——. Principal genus, Crassatella.
Fam. 4. Cardiniidae.—Shell equivalve, oval or triangular, ligament external, cardinal teeth small, laterals fairly strong. Devonian——Oolite. Principal genera: Cardinia, Anthracosia, Carbonicola, Anoplophora.
Fam. 5. Cyprinidae.—Anal and branchial orifices complete, papillose, foot thick; shell variable, equivalve, thick, umbones often spiral, hinge-teeth very variable, ligament external. Jurassic——. Principal genera: Cyprina; Pygocardia (Crag), Veniella (Cretaceous), Venilicardia (Secondary strata), Anisocardia (Jurassic), Isocardia, Libitina, Coralliophaga; Basterotia (Eocene). The families Pachydomidae (Palaeozoic) and Megalodontidae (Palaeozoic—Secondary) are probably related to the Cyprinidae.
Fig. 305.—Isocardia vulgaris Reeve, China.