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.
Fam. 1. _Arcidae._--Borders of the mantle bear compound pallial
eyes. The labial palps are direct continuations of the lips. Hinge
pliodont, that is to say, it has numerous teeth on either side of
the umbones and the teeth are perpendicular to the edge. _Arca_;
foot byssiferous; British. _Pectunculus_; foot without byssus;
British. _Scaphula_; freshwater; India. _Argina. Bathyarca.
Barbatia. Senilia. Anadara. Adacnarca._
Fam. 2. _Parallelodontidae._--Shell as in _Arca_, but the posterior
hinge teeth elongated and parallel to the cardinal border.
_Cucullaea_; recent and fossil from the Jurassic. All the other
genera are fossil: _Parallelodon_; Devonian to Tertiary.
_Carbonaria_; Carboniferous, &c.
Fam. 3. _Limopsidae._--Shell orbicular, hinge curved, ligament
longer transversely than antero-posteriorly; foot elongate, pointed
anteriorly and posteriorly. _Limopsis. Trinacria_; Tertiary.
Fam. 4. _Philobryidae._--Shell thin, very inequilateral, anterior
part atrophied, umbones projecting. _Philobrya._
Fam. 5. _Cyrtodontidae._--Extinct; shell equivalve and
inequilateral, short, convex. _Cyrtodonta_; Silurian and Devonian.
_Cypricardites_, Silurian. _Vanuxemia_; Silurian.
Fam. 6. _Trigoniidae._--Shell thick; foot elongated, pointed in
front and behind, ventral border sharp; byssus absent. _Trigonia_;
shell sub-triangular, umbones directed backwards. This genus was
very abundant in the Secondary epoch, especially in Jurassic seas.
There are six living species, all in Australian seas. Living
specimens were first discovered in 1827. _Schizodus_; Permian.
_Myophoria_; Trias.
Fam. 7. _Lyrodesmidae._--Extinct; shell inequilateral, posterior
side shorter; hinge short, teeth in form of a fan. _Lyrodesma_;
Silurian.