146 DIM Y ARIA. 



Distinguished from the genera Venus, Cytherea, and Ai'themis, by 

 having one impressed lateral tooth, on the front side, which is some- 

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

 and terminated near the apices by a carity, 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 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 spurious epidermis. 



Genus 2Q GALATHEA — Lamarck. 



Generic Character. — Equivalve, subtrigonal, covered with 

 a greenish epidermis ; two furrowed primary teeth in the 

 right valve, joined at their base ; aiul 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 Cyreua, but distinguislied by the divej'gent form 

 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 and 

 a half inches long. Inhabits the rivers of Ceylon. 



