CONCHIFERA. 



265 



equal, turning either to the right or left ; one thick and 

 long tooth in one valve, and a deep pit in the other ; liga- 

 ment external. Animal, with the two folds of the mantle 

 for the most part separated ; behind they are united, and 

 form two very short siphons, whose edges are strongly 

 beset with cirrhi ; the foot is small, cylindrical, truncated, 

 and bent in the form of a knee ; the mouth is small, and 

 has on each side a pair of almost four-sided, crookedly 

 truncated tentacles. — 55 species* ; also fossil. 



These shells appear to present almost every variety of 

 shape and colour ; the former fact is perhaps owing to 

 the accidents of their position, as they are found usually 

 at small depths in the sea, affixed by the larger valve 

 to rocks, &c, and occasionally united together in large 

 masses. They are so firmly attached that it is difficult 

 to avoid breaking the shell when they are removed. 

 They are confined to warm climates, the Mediterranean 

 being the lowest temperature. 



Diceeas. Lam. — Shell inequivalve ; bosses very much 

 lengthened, distorted, and turned different 

 ways, attached to substances by the point 

 of the larger boss ; one large tooth in each 

 valve; pallial impression without sinus. — 2 



Species fossil. Diceras. 



These curious shells are found only in the fossil state, 

 and are of very irregular form ; the two almost spirally 

 twisted bosses looking like a pair of horns; and the 

 larger of the two bearing the appearance of having been 

 attached to other substances. 



Chamostr^a. Roissy. (Cleidothaerus Stutehbury.) 

 — Shell inequivalve, and somewhat pearly in the interior ; 



* Reeve' s Iconica. 



