MARINE MOLLUSCA OF THE UNITED STATES. 141 



Synopsis of Genera. 

 * Ligament external. 



Shell ovate, oblong or rounded posteriorly ; beaked or angular, with a 

 flexuosity on the hind slope. One or two primary teeth in each valve ; 

 lateral teeth present or obsolete. Tellixa. 



Shell orbicular, convex, surface of valves divaricately striated ; posterior 

 flexure obsolete. A small anterior and a large bifid cardinal tooth in the 

 right valve, and a single cardinal tooth in the left valve ; lateral teeth 

 two in each valve. Strigilla. 



Shell oval, convex or sub-ventricose. Cardinal teeth small; no lateral 

 teeth. Macoma. 



Shell triangular, very inequivalve, the right valve concave ; surface of 

 valves plicate ; beaks acute, laterally incurved ; lateral slopes strongly 

 produced and dentate at their edges. Hinge with two primary teeth in 

 one valve and one in the other ; lateral teeth two in each valve. 



Tellidora. 



Shell oblong or irregular ; two cardinal teeth in the right valve, the poste- 

 rior one thin and directed obliquely backwards ; two cardinal teeth in 

 the left valve, the posterior one stout, bilobed, the anterior one smaller. 

 No distinct lateral teeth. Gastranella. 



Shell more or less wedge-shaped, equivalve ; the hinder side much shorter 

 than the anterior. Two cardinal teeth in one valve and one bifid tooth 

 in the other ; one or two lateral teeth in each valve. Donax. 



* Cartilage internal, situated in a cartilage pit. 



Shell thin, transversely elongated, slightly gaping at the sides ; surface 

 smooth, covered with a thin, deciduous epidermis. Cartilage pit oblique ; 

 primary teeth small or wanting ; lateral teeth distinct ; ligament short, 

 partly external. Abra. 



Shell transversely oval or orbicular, slightly gaping at the sides. Hinge 

 with one or two primary teeth in each valve, with a long narrow pit be- 

 tween them for the internal cartilage ; ligament external, thin. 



Semele. 



Shell equivalve, inequilateral, anteriorly rounded, posteriorly subtruncate 

 and slightly gaping. Hinge with a small, anterior primary tooth in each 

 valve ; cartilage internal, in a spoon-shaped cavity projecting into the 

 cavity of the valves ; one strong lateral tooth on each side of the hinge 

 in one valve, no lateral teeth in the other. Cumingia. 



Shell ovate cuneate, truncated behind. Hinge with a simple, compressed 

 primary tooth and a rudimentary process in the place of the second tooth ; 

 lateral teeth sub-equal, compressed, strongly cross-grooved. Ceronia. 



Shell equivalve, inequilateral, oblong, closed ; surface nearly smooth, or 

 concentrically striated. Two diverging teeth in each valve, one of them 

 in the right valve, elevated and conspicuous ; cartilage in a pit in each 



