140 DIMYARIA. 



valve, subequilateral, and close ; hinge arcuated, teeth nu- 

 merous, oblique, alternately inserted into the opposite valves, 

 middle teeth obsolete ; ligament external. 



This differs from the other genera of many toothed shells, in having 

 the ligament partially inserted internally, and in being destitute of the 

 angular groove on the exterior of the valves. 



Section * No distinct longitudinal furrows. ** With raised longi- 

 tudinal radiating ribs. 



Pectunculus glycimeris The Delicious Pectunculus. 



Plate XVI. fig. 8. Suborbicular ; finely striated trans- 

 versely and longitudinally ; covered with a villous skin, 

 under which it is marked with reddish-chestnut spots or 

 bands ; inside white, and the margin crenulated. From two 

 to three inches long. Inhabits the British and Mediter- 

 ranean seas. 



Genus 12. — ARC A. — Linnoeus. 



Generic Character. — Shell transverse, subequivalve, in 

 equilateral; beaks remote, separated by the area of the liga- 

 ment; hinge linear, straight, without ribs at the extremities; 

 teeth of the hinge numerous, serrated, closely set, alternately 

 inserted ; ligament external. 



Section * Lower margin not crenated. ** Lower margin internally 

 crenated. 



Area Noce. — Noah's Ark. Plate XVI. fig. 9. Oblong, 

 striated transversely, and longitudinally ribbed ; umbones 

 remote, and incurvated ; margin entire and gaping ; cream- 

 white, with divergent and zigzag chestnut stripes ; inside 

 white. Two inches broad. Inhabits the American and 

 British seas. 



Genus 13.— CUCULLiEA.— Zawwrcy^. 



Generic Character. — Shell equivalve, inequilateral, tra- 

 peziform, ventricose ; beaks distant, separated by the area 

 of the ligament ; anterior muscular impression forming a 

 prominence with the margin angulated, or with an eared 

 produced border; hinge linear, straight, with very &mall 



