DICERAS. 



the larger valve; " one very large, thick, concave, somewhat 

 ear-shaped tooth is prominent in the larger valve;'' there 

 are two lateral, distant, muscular impressions in each 

 valve, and the umbones are large, very prominent, divari- 

 cated and rather irregularly spirally twisted, having a 

 good deal of the appearance of a pair of horns; the generic 

 name of Diceras has reference to this character of the 

 umbones resembling two horns, while that of arietinum, 

 by which Lamarck has designated the only species known 

 to him, refers to the peculiar twisted character of rams 

 horns by which the contorted umbones are characterized. 



Only known in a fossil state, and found near Geneva 

 and in Normandy ; the casts are in granular Limestone, 

 and the remains of the shell, when they occur, consist of 

 crystallized Calcareous spar. 



