278 



MANUAL OF THE MOLLUSCA. 



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



p, point of attachment; I, 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. 



Fig. ISS. Diceras arietinum, a. Fig. 189. Requienia ammonia, I. 



a, point of attachment; I, I, ligamental grooves ; t, posterior adductor ii.fleciion. 



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, teeth 

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

 times obsolete. 



Distr. 5 sp. Middle oolite. Germany, S\^'itz. France, Algeria. 



Diceras difi'ers from Chama in the great prominence of both its imlones, 

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

 and in the prominent ridges bordering the muscular impressions. Similar 

 ridges exist in Cucullcea, Megalodon, Cardilia and the Hippurite ; they pro- 

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

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

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



