Two pallial sutures. Siphons generally short. Foot cylindrical, more
or less elongated, byssogenous. Gills much folded. Shell equivalve,
with radiating costae and external ligament.
Fam. 1. _Cardiidae._--Mantle slightly closed; siphons very short,
surrounded by papillae which often bear eyes; foot very long,
geniculated; pallial line without sinus; two adductors, _Cardium_;
British. _Pseudo-kellya. Byssocardium_; Eocene. _Lithocardium_;
Eocene.
Fam. 2. _Limnocardiidae._--Siphons very long, united throughout;
shell gaping; two adductors; brackish waters. _Limnocardium_;
Caspian Sea and fossil from the Tertiary. _Archicardium_; Tertiary.
Fam. 3. _Tridacnidae._--Mantle closed to a considerable extent;
apertures distant from each other; no siphons; a single adductor;
shell thick. _Tridacna. Hippopus._