150 DIMYARIA. 



and separated from the cardinal teeth, similar to those of Mactra, 

 Lucina, and TeUina. In the genera Cytherea and Venus, the shortest 

 side of the sheU is always the posterior : and the longest and largest, 

 having the ligament attached to it, is the anterior ; while in Donax, 

 this structure is reversed, the ligament being affixed to the shortest 

 side as in the genus Tellina. 



Donax irunculus.—THE Truncated Donax. Plate 

 XVII. fig. 5. Oblong, glossy, finely striated, and radiated 

 longitudinally; transversely banded with purple; white and 

 clouded with purple within ; margin crenulated. One inch 

 and a quarter broad. Inhabits the seas of Europe. 



This genus consists of two sections : — * Shells with the margins en- 

 tire. — ** Margins crenated. 



Genus 33.— LUCINA.— ZawarcA. 



Generic Character. — Suborbicular, inequilateral; beaks 

 small, pointed, and oblique ; hinge variable, sometimes with 

 two divergent teeth, one of which is bifid, but changing with 

 age ; two lateral teeth, sometimes obsolete ; the posterior 

 nearest the primary ones ; two distinct muscular impressions 

 widely separated, the posterior one prolonged. 



In the structure of the hinge, Lucina is allied to Tellina, but although 

 in many instances there is a distinctly marked angular depression on 

 the shell, it never possesses the irregular plait or fold charactei-istic of 

 the Tellina;. 



Lucina undata. — The Waved Lucina. Plate XVII. 

 fig. 7. Orbicular, thin, convex, undulated with fine irregular 

 striae; pale straw-coloured yellow, and white in the inside; 

 margin glossy and plain. Inhabits the British seas. 



Genus 34. -MY SI A.— Zeac/*. 



Generic Character. — Suborbicular, inequilateral; beaks 

 small, pointed, and very slightly turned to one side ; hinge 

 with two primary teeth, joined at their base and divergent, 

 behind which is an oblique descending cavity; with one 

 strong muscular impression in each valve ; margin thin and 

 entire ; ligament external. 



