DIMYAIRA. ]37 



and transversely oblong ; upper valve flattened, and the under 

 one convex ; hinge with two oblong diverging cardinal teeth, in 

 the upper valve, and the other valve with two oblong grooves ; 

 ligament external. 



The hinge somewhat like the Placuna, but more nearly allied to the Cor- 

 bula, haviug two muscular impressions. 



Pandora rostrata. — The Beaked Pandora. Plate XVII. 

 fig. 19. White, oblong, much produced towards the beak ; one 

 valve nearly flat, the other convex ; rounded at the anterior end. 

 One inch broad. Inhabits the Mediterranean and British seas. 



Genus m.— CORBUhK.—Bruguiere. 



Generic Character. — Shell regular, inequi valve, inequilateral ; 

 with a conical, bent, ascending, primary tooth in each valve ; a 

 small pit at its side ; destitute of lateral teeth ; ligament ex- 

 ternal. 



Although having some affinity to the genera Crassatella, and Ungulina, 

 they are at once distinguished by the inequality of their valves, and the 

 strong primary elevated tooth. 



Corhula nucleus. The Kernel Corbula. Plate XVII. 

 fig. 20. Somewhat triangular, strong, with the under valve much 

 larger than the upper one ; transversely striated ; covered with 

 a thick, brown epidermis. Half an inch long. Inhabits the 

 British seas. 



TRIBE IV. MACTRACEA. 



Equivalve shells, frequently gaping at the lateral extremities ; 

 with an internal ligament, or partly external, and the animal 

 having a small compressed foot. 



* Ligament seen externally or double. 



Gem<s47.— AMPHIDESMA Lamarck. 



Generic Character. — Inequilateral, transverse, suboval or 

 somewhat rounded ; with sides slightly gaping ; hinge with one 

 or two cardinal teeth, and a narrow groove for the internal liga- 

 ment ; hgament double, the external one short, the internal one 

 fixed in the internal grooves. 



These possess a peculiar character which distinguishes them from all 

 other bivalves, namely the two ligaments. 

 U 3 



