146 DIMYARIA. 



Distingviished from the genera Venus, Cytherea, and Arthemis, by 

 having one impressed lateral tooth, on the front side, which is som.e- 

 times obsolete ; the nymphae or callosities of the hinge large, arched, 

 and terminated near the apices by a cavity, sometimes very deep. 



Cyprina Islandica The Islandic Cyprina. Plate 



XVI. fig. 22. Suborbicular, convex, strong, irregularly 

 striated ; covered with a deep black-brown epidermis ; white 

 within. Three and three-fourth inches long. Inhabits the 

 Atlantic ocean and British seas. 



Genus 25.— LAS^A Leach. 



Generic Character. — Equivalve, inequilateral ; hinge not 

 quite central ; umbo prominent ; hinge with two nearly ob- 

 solete primary teeth, lateral ones very conspicuous ; valves 

 convex ; margin plain. 



Lascea rubra — The Reb Lasjea. 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 spurious epidermis. 



Genus 26 GALATHEA Lamarck. 



Generic Character. — Equivalve, subtrigonal, covered with 

 a greenish epidermis ; two furrowed primary teeth in the 

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

 the intermediate one being separate and protruding; liga- 

 ment external, short, turgid, and prominent ; the elongated 

 mark protruding in the centre. 



Somewhat allied to CjTcna, but distinguished by the divergent fonii 

 of its primary tooth. They all inhabit fresh waters. 



Galathea radiata — The Rayed Galathea. Plate 

 XVI. fig. 20. Somewhat trigonal, gibbous towards the 

 base ; covered with a yellowish-green thin epidermis, be- 

 neath which it is radiated with pale chestnut. Three aud 

 a half inches long. Inhabits the rivers of Ceylon. 



