218 MOLLUSCA. LAMELLIBRANCHIA 



lower part of the posterior border. Surface nearly smooth or with 

 radial ribs. Right valve with one, sometimes two, cardinal teeth, and 

 ridge-like posterior lateral tooth. Left valve with a triangular, some- 

 times bifid cardinal, and one anterior and one posterior lateral tooth. 

 Adductor impressions with a ridge passing to the hinge. Trias and 

 Rhsetic. Ex. M. laevigata, Trias. 



Unio. Shell thick, oval or elongated, with a thin periostracum. 

 Surface smooth, tuberculate, striated, or folded ; interior nacreous. 

 Umbones more or less anterior, often corroded. Ligament external, 

 elongated. In the right valve one or two thick, irregular teeth 

 below the umbo and a long lamellar posterior lateral tooth ; in the 

 left valve, two thick irregular teeth near the umbo, and two long 

 lamellar posterior lateral teeth. Adductor impressions very deep, 

 especially the anterior. Pallial line simple. Inferior Oolite to 

 present day. Lives in fresh water. Ex. U. littoralis, Pleistocene 

 and Recent. 



Anodonta. Allied to Unio ; shell relatively thin, without 

 teeth. Fresh water. Miocene (perhaps Purbeck) to present day. 



Carbonicola (= Anthracosia). Similar in form to Unio, but 

 the anterior part of shell is broad and tumid, the posterior part 

 narrow and compressed ; usually a constriction at the ventral border. 

 Hinge-plate triangular, with or without cardinal teeth, no laterals. 

 Adductors large, the anterior near the margin. Pedal impression 

 above the anterior adductor. Carboniferous and Permian. Probably 

 fresh water. Ex. C. robusta, Coal Measures. 



"j 



Anthracomya. Differs from Carbonicola chiefly in having 

 the posterior part of the shell broad and expanded. Hinge-plate 

 small, with a cardinal and one posterior lateral tooth. Carboniferous. 

 Probably fresh water. Ex. A. adamsi. 



Cardinia. Shell trigonal, oval, or oblong, very inequilateral, 

 compressed, thick, marked by lines of growth. Interior not na- 

 creous. Umbones small, sharp, close together. Ligament external. 

 Cardinal teeth small or obsolete ; in the right valve one anterior 

 lateral tooth, in the left, one posterior lateral. Impression of anterior 

 adductor very deep. Pallial line simple. Trias to Middle Jurassic 

 (chiefly Lias). Ex. C. listeri, Lias. 



