CHAMID. 199 
Shell attached by the dextral umbo; valves alike in struc- 
ture and sculpturing; fixed valve straight, inversely conical, 
with a long, straight lizamental groove, and obscure hinge-area ; 
opercular valve flat or convex, with an oblique, submarginal 
umbo. 
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. 
VALLETIA, Munier-Chalmas, 1872. V. Tombecki, M.-Ch. Neo- 
comian ; Savoy. 
Diceras, Lamarck, 1804. 
Syn.—Heterodiceras, Munier-Chalmas. Pseudodiceras, Gem- 
mellaro. 
Distr.—5 sp. Middle Oolite ; Germany, Switzerland, France, 
Algeria. D. arietinum, Lam. (exvii, 5-7). 
Shell subequivalve, attached by either umbo; beaks very 
prominent, spiral, furrowed externally by ligamental 2rooves ; 
hinge very thick, teeth 2°1, prominent; muscular impressions 
bounded by long spiral ridges, sometimes obsolete. 
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. Similar ridges exist in Cucullea, 
Megalodon, Cardilia, and Hippurites ; 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 are 
frequently obsolete. The dental pits are much deeper than the 
teeth which they receive, and are subspiral, giving rise to bifid 
projections on the casts; the single tooth in the left valve con- 
sists of two elements, and the cavity fosset) which receives it is 
divided at the bottom. 
Munier-Chalmas divides Diceras thus: 
DICERAS, Lam., 1805. Posterior muscular impression on a plate, 
more or less elevated, the anterior upon plate-like ridges. D. 
arietinum, Lam. 
HETERODICERAS, Munier-Chalmas, 1869. Posterior impression 
in each valve upon horizontal or concave surfaces, connected to 
the cardinal plate, anterior impression upon similar surfaces, 
more or less horizontal. D. Lucii, Defrance. 
PLESIODICERAS, Munier-Chalmas, 1882. Postero-cardinal tooth 
more or less prolonged anteriorly, and reversed behind ; posterior 
muscle advancing a little between the cardinal margin and the 
base of the postero-cardinal tooth. D. Valfinense, Boehm. 
BAYLEIA, Munier-Chalmas, 1872. One valve resembling Diceras 
by its raised plate for the muscular impression, the other 
approaching caprinoid groups by its beak and by the two cavities 
