In addition to the characters given above, it may be noted that the mantle is provided with a hypobranchial gland on the outer side of each gill, the auricles are muscular, the kidneys are glandular through their whole length, the sexes are separate.
Fam. 1. Solenomyidae.—One row of branchial filaments is directed dorsally, the other ventrally; the mantle has a long postero-ventral suture and a single posterior aperture; the labial palps of each side are fused together; shell elongate; hinge without teeth; periostracum thick. Solenomya.
Fam. 2. Nuculidae.—Labial palps free, very broad, and provided with a posterior appendage; branchial filaments transverse; shell has an angular dorsal border; mantle open along its whole border. Nucula. Acila. Pronucula.
Fam. 3. Ledidae.—Like the Nuculidae, but mantle has two posterior sutures and two united siphons. Leda. Yoldia. Malletia.
Fam. 4. Ctenodontidae.—Extinct; Silurian.
The fossil group Palaeoconcha is connected with the Protobranchia through the Solenomyidae. It contains the following extinct families.
Fam. 1. Praecardiidae.—Shell equivalve with hinge dentition as in Arca. Praecardium; Silurian and Devonian.
Fam. 2. Antipleuridae.—Shell inequivalve. Antipleura; Silurian.
Fam. 3. Cardiolidae.—Shell equivalve and ventricose; hinge without teeth. Cardiola; Silurian and Devonian.