MARINE MOLLUSCA OF THE UNITED STATES. 147 
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. TELLINA. 
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 
