CONCHIFERA. 439 
Dicreras, Lamarck. 
Type, D. arietinum, Pl. 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. Germany, Switzer- 
land, France, Algeria. 
Diceras differs from Chama in the great prominence of both 
its wmbones, in haying constantly two hinge-teeth in the right 
( Md 
ii il 
wa 
lh 
1 
WT of 
1 
i 
ies i 
\ 
RMN \ 
Fig. 227. Diceras arietinum, 3. Fig. 228. Requienia ammonia, }. 
a, point of attachment ; /, 7, 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 Cucullea, 
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 (/osset) which receives it is divided at the bottom. 
REQUIENIA, Matheron, 
Dedicated to M. Requien, author of a Catalogue of Oorsivan 
Mollusca. 
Example, R. Lonsdali, Pl. XVIII., Fig. 12 and Fig. 230. 
R. ammonia, Fig. 189. 
Shell thick, very inequivalve, attached by the /e/t umbo; liga- 
meut external; teeth 2:1; left valve spiral, its cavity deep, 
