334 



POPULAR CONCHOLOGY. 



Family 2. — THECIDEAD^. 



The arms of the animal are entirely united in the form 

 of two or more lobed processes, which are sunk into grooves 

 on the convex inner surface of the ventral valve. 



Megathyris. D^Orb. (Argyope Deslong champs.) — 

 Shell inequivalve, inequilateral, semi-orbicular, compressed, 

 often strong, surface smooth or with radiating 

 ribs, always punctured, area large, triangular, 

 with a large incomplete foramen under the 

 beak of the upper valve, and impinging on , 

 the lower; no deltidium; no free apophy- 

 sary system, but one or more ribs or dissepiments, arising 

 from the inner surface of the smaller valve. Animal with 

 contorted or spiral arms fixed to the margin of the apo- 

 physary ribs or cardinal teeth.* — Few species ; also fossil. 



Mr. Forbes remarks that (( the few living species known 

 of this interesting genus are inhabitants of deep — of very 

 deep — water." M. cistellata was formerly only known 

 in the fossil state, but a few specimens have been lately 

 procured, attached to stones at great depths, off" the coast 

 of Scotland. Philippi remarks that M. (Anomia) detrun- 

 cata, of Grmelin, is the type of the genus, and is found in 

 the Mediterranean. 



Thecidium. Def ranee. — Shell ovate, small, 

 inequivalve, nearly equilateral, imperforate, and 

 attached by the concave valve, which has two 

 short cardinal processes, and a rather irregular 

 area, extending from the hinge to the boss; 

 the smaller valve is rather flat, and has a small 



Thecidium 

 mcditcrro u< a. 



* Forbes's British Moll. 



