278 



MANUAL OP THE MOLLUSCA. 



Fig. 186. Bi-radiolites,%- Fig. 187. Monopleura, . 



p, point of attachment ; /, ligamental groove; a, a, corresponding areas. 

 Fossil, 9 sp. Neocomian Chalk. France, Texas. They are commonly 

 found in groups, adhering laterally, or rising one above the other ; the casts of 

 such as are known are quite simple and chama-like. 



a 



Fig. 188. Diceras arietinum, f. Fig. 189. Requienia ammonia, 



a, point of attachment; /, I, ligamental grooves ; t, posterior adductor inflection. 



DICERAS, Lamarck. 



Type, D. arietinum, PI. XVIII. figs. 10, 11, and fig. 188, 190. 



Shell sub-equivalve, attached by either umbo ; beaks very prominent 

 spiral, furrowed externally by ligamental grooves ; hinge very thick, teet 

 2.1, prominent; muscular impressions bounded by long spiral ridges, some 

 times obsolete. 



Distr. 5 sp. Middle oolite. Germany, Switz. France, Algeria. 



Diceras differs from Chama in the great prominence of both its umbones 

 in having constantly two hinge-teeth in the right valve and one in the left 

 and in the prominent ridges bordering the muscular impressions. Simila 

 ridges exist in Cucull&a, Megalodon, Cardilia and the Hippurite ; they pro 

 duce deep spiral furrows on the casts, which are of common occurrence in th 

 Coral-oolite of the Alps. One or both the anterior furrows (fig. 190, t, t) ar 

 frequently obsolete. The dental pits are much deeper than the teeth whic 



