CONCHIPEEA. 



DiCEEAS, Lamarck. 



439 



Ttjpe, D. arietinum, PI. XVIII., Figs. 10 and 11, and Figs. 

 227 and 228. 



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, sometimes obsolete. 



Distribution, 5 species. Middle oolite. Grermany, Switzer- 

 land, France, Algeria. 



Diceras differs from Chama in the great prominence of both 

 its umbones, in having constantly two hinge-teeth in the right 



Fig. 227. Diceras arietinum, f, Fig. 228. Requienia ammonia, J. 



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



valve and one in the left, and in the prominent ridges bordering 

 the muscular impressions. Similar ridges exist in CucuUcsa, 

 Megalodon, Cardilia, and the Hippurite ; they produce deep 

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

 the Coral-oolite of the Alps. One or both the anterior furrows 

 (Fig. 229, t, t) are frequently obsolete. The dental pits are 

 much deeper than the teeth which they receive, and are sub- 

 spiral, giving rise to bifid projections (c, c) on the casts ; the 

 single tooth in the left valve consists of two elements, and the 

 cavity [fosset) which receives it is divided at the bottom. 



EEQUiEisriA, Matheron, 



Dedicated to M. Eequien, author of a Catalogue of Corsican 

 MoUusca. 



Example, E. Lonsdalii, PI. XYIII., Fig. 12 and Fig. 230. 

 E. ammonia, Fig. 189. 



Shell thick, very inequivalve, attached by the left umbo ; liga- 

 ment external ; teeth 2:1; left valve spiral, its cavity deep, 



