132 DIMYAIRA. 



tarhed to it, is the anterior ; while in tlie Donax, this stnicturo is reversed, 

 the ligament being affixed to the shortest side as in the genus Tellina. 



Donax trunculus. — The Truncated Donax. Plate XVII. 

 fig. 5. Oblong, glossy, finely striated, and radiated longitudi- 

 nally ; 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 mar- 

 gins entire, — ** Margins crenated. 



Genus 33.— LUCINA.— ZomarcA. 



Generic Character. — Suborbicular, inequilateral ; beaks small, 

 pointed, and oblique; hinge variable, sometimes with two diver- 

 gent teeth, one of which is bifid, but changing with age ; two 

 lateral teeth, the intermediate one obsolete ; the posterior near- 

 est the primary ones ; two distinct muscular impressions widely 

 separated, the posterior one prolonged. 



Having lateral teeth, the Lucina is allied to Tellina, but although in many 

 instances they possess a distinctly marked angular depression on the shell, it 

 is never flexuous. 



Lucina undata. — The Waved Lucina. Plate XVII. fig. 7. 

 Orbicular, thin, convex, undulated with tine irregular strite ; pale 

 straw-coloured yellow, and white in the inside •, margin glossy 

 and plain. Inhabits the British seas. 



Genus Si.— MY SI A.— Leach. 



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 im- 

 pression in each valve ; margin thin and entire ; ligament ex- 

 ternal. 



Mysia rotundata. — The Rounded Mysia. Plate XVII. 

 fig. 6. Orbicular,somewhat convex, thin, sub-pellucid, and ob- 

 scurely striated ; umbones small and oblique. Three quarters 

 of an inch long. Inhabits the British seas. 



