Fam. 4. Grammysiidae.—Shell thin, equivalve, oval or elongate; hinge without teeth. Grammysia; Silurian and Devonian. Protomya; Devonian. Cardiomorpha; Silurian to Carboniferous.
Fam. 5. Vlastidae.—Shell very inequivalve; hinge without teeth. Vlasta; Silurian.
Fam. 6. Solenopsidae.—Shell equivalve, greatly elongated, umbones very far forward. Solenopsis; Devonian to Trias.
Order II. Filibranchia
Gill-filament ventrally directed and reflected, connected by ciliated junctions. Foot generally provided with a highly developed byssogenous apparatus.
Sub-order I.—Anomiacea.
Very asymmetrical, with a single large posterior adductor. The heart is not contained in the pericardium, lies dorsad of the rectum and gives off a single aorta anteriorly. The reflected borders of the inner gill-plates of either side are fused together in the middle line. The gonads open into the kidneys and the right gonad extends into the mantle. Shell thin; animal fixed.
Fam. 1. Anomiidae.—Foot small; inferior (right) valve of adult perforated to allow passage of the byssus. Anomia; byssus large and calcified; British. Placuna; byssus atrophied in adult. Hypotrema. Carolia. Ephippium. Placunanomia.
Sub-order II.—Arcacea.
Symmetrical; mantle open throughout its extent; generally with well developed anterior and posterior adductors. The heart lies in the pericardium and gives off two aortae. Gills without interlamellar junctions. Renal and genital apertures separate.