Two pallial sutures, siphons somewhat elongated and partially or
wholly united. Gills slightly folded. A bulb on the posterior aorta.
Ligament external.
Fam. 1. _Veneridae._--Foot well developed; pallial sinus shallow or
absent. _Venus_; British. _Dosinia_; British. _Tapes_; British.
_Cyclina. Lucinopsis_; British. _Meretrix. Circe_; British.
_Venerupis._
Fam. 2. _Petricolidae._--Boring forms with a reduced foot; shell
elongated, with deep pallial sinus. _Petricola. P. pholadiformis_,
originally an inhabitant of the coast of the United States, has been
acclimatized for some years in the North Sea.
Fam. 3. _Glaucomyidae._--Siphons very long and united; foot small;
shell thin, with deep pallial sinus; fresh or brackish water.
_Glaucomya. Tanysiphon._