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.
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.