CO^CRYU \—DITHYRA. 56. 247 



Tab. nost. 16, fig. 2. 

 Mya ovata. Donovan, British Sliells, iv. <ab. 122. 



Mus. nost. From the Avon, near Bristol. 



Shell two inches long, three and a half broad, and nearly as 

 deep as it is long, covered with a blackisli-brown skin, under 

 which it is of the richest pearly lustre ; thick and ponderous 

 with the margin thick and very obtuse, nearly cylindrical about 

 the hinge, from which it regularly tapers in a conical manner to 

 the anterior side: beaks rugged, and often decorticated. 



Montagu, at p. 36, is of opinion, that this is only a variety of 

 Mysca ovata : but a series of each of them, from the Avon and 

 the Froome, has sufficiently convinced us of their specific distinc- 

 tion. One is uniformly flat and thin; the other is nearly cylin- 

 drical and thick. 



56. CyCLkS— B RUG UIERE. 



Testa orbicularis vel ovata, aquicalvis, clausa. Cardo den- 

 tibus duobus minutis divergcntibus : lateralibus utrinqne, 

 valvfP alteriiis didymis. Ligamentum externum. 



Shell orbicular or oval, equivalve, closed. Hinge with two 

 minute divergent teeth : and lateral ones on each side, which 

 are double in one of the valves. 



