128 DIMYAIRA. 



terminating in the margin ; inside vvliite. Two inches long. 

 Inhabits the European seas. 



Genus 21.— CYPRINA— Zcmarc^. 



Generic Character — Shell equi valve, inequilateral, obliquely 

 heart-shaped, beaks obliquely bent ; hinge with three unequal 

 teeth, approximate at the base, and slightly divergent above ; 

 lateral cardinal tooth remote, which is sometimes obsolete ; liga- 

 ment external, and partly sunk between the beaks. 



Distinguished from tlie genera Venus, Cytherea, and Exoleta, by having 

 one impressed lateral tooth, on the front side, which is sometimes obsolete ; 

 the nymphse or callosities of the hinge large, arched, and terminated near 

 the apices by a cavity, sometimes very deep. 



Cyprina Islandica. — The Islandic Cvprina. Plate XVL 

 fig. 22. Suborbicular, convex, strong, irregularly striated j cov- 

 ered with a deep black-brown epidermis ; white within. Three 

 and three-fourth inches long. Inhabits the Atlantic ocean and 

 British seas. 



Genus 25.— LA S^ A.— XeacA. 



Generic CAarac/er. — Equivalve, inequilateral ; hinge not quite 

 central; umbo prominent ; hinge with two nearly obsolete pri- 

 mary teeth, lateral ones very conspicuous ; valves convex ; mar- 

 gin plain. 



Lascea rubra.— The Red Las^a. Plate XVI. fig. 16. 

 Convex, smooth, glossy, pellucid, reddish-pink. Fourth of an 

 inch long. Inhabits the British seas. 



Section II. — Fluviatile. 



Shells with lateral teeth at the hinge, and covered with a spu- 

 rious epidermis. 



Genus 26.— G Ah ATHE A. —Lamarck. 



Generic Character. — Equivalve, subtrigonal, covered with a 

 vitreous greenish epidermis ; two furrowed primary teeth in the 

 right valve, joined at their base ; and three in the other, the in- 

 termediate one being separate and protruding ; ligament external, 



