CHAMID^. 199 



Shell attached by the dextral umbo ; valves alike in struc- 

 ture and sculpturing ; fixed valve straight, inversely conical, 

 with a long, straight ligamental groove, and obscure hinge-area; 

 opercular valve flat or convex, with an oblique, submarginal 

 umbo. 



The}^ are commonly found in groups, adhering lateral!}^, or 

 rising one above the other ; the casts of such as are known are 

 quite simple and Chama-like. 



VALLETTA, Munier-Chalmas, 1872. V. Tomhecki, 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. (cxvii, 5-7). 



Shell subequivalve, attached b}^ either umbo ; beaks \Qvy 

 prominent, spiral, furrowed externally by ligamental grooves ; 

 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 CucuUsea, 

 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 \alve con- 

 sists of two elements, and the cavity i^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 

 injeach valve upon horizontal or concave surfaces, connected to 

 the cardinal plate, anterior impression upon similar surfaces, 

 more or less horizontal. D. Lucii^ Defrance. 



PLESIODICERAS, Muuicr-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. Valjinense, 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 



