Symmetrical, the anterior adductor small or absent. Heart gives off
only an anterior aorta. Surface of gills smooth, gill-filaments all
similar, with interlamellar junctions. Gonads generally extend into
mantle and open at sides of kidneys. Foot linguiform and byssiferous.
Fam. 1. _Mytilidae._--Shell inequilateral, anterior end short; hinge
without teeth; ligament external. Mantle has a posterior suture.
Cephalic eyes present. _Mytilus_; British. _Modiola_; British.
_Lithodomus. Modiolaria_; British. _Crenella. Stavelia. Dacrydium.
Myrina. Idas. Septifer._
Fam. 2. _Modiolopsidae._--Extinct; Silurian to Cretaceous; adductor
muscles sub-equal. _Modiolopsis.--Modiomorpha. Myoconcha._
Fam. 3. _Pernidae._--Shell very inequilateral; ligament subdivided;
mantle open throughout; anterior adductor absent. _Perna.
Crenatula_; inhabits sponges. _Bakewellia. Gervilleia_; Trias to
Eocene. _Odontoperna_; Trias. _Inoceramus_; Jurassic to Cretaceous.