200 CHAMIDA. 
for the insertion of the anterior muscle. D. Powechi, Munier- 
Chalmas. Cret.; France. 
REQUIENIA, Matheron, 1842. 
Dedicated to M. Requien, author of a Catalogue of Corsican 
Mollusca. 
Distr.—Fossil, Tsp. Neocomian—L. Chalk; Britain, France, 
Spain; Algeria, Texas. R. Lonsdalii, Sowb. (exvii, 8, 9, 12). 
R. ammonia, (exvii, 10, 11). 
Shell thick, very inequivalve, attached by the left umbo; liga- 
ment external; teeth 2°1; left valve spiral, its cavity deep, not 
concamerated ; free valve smaller, subspiral ; posterior adductor 
bordered by a prominent subspiral ridge in each valve. 
The shell-structure of Requienia is like Chama. The relative 
size of the valves is subject to much variation; in R. Favri 
(Sharpe) they are nearly equal. The hinge-teeth are like those 
of Diceras; the cavity for the posterior tooth of the right valve 
is very deep and subspiral. The internal muscular ridges. are 
produced by duplicatures of the shell-wall, and are indicated 
outside by grooves. In &. subequalis and Toucasiana there is 
a second parallel ridge, as in Hippurites and Caprotina. 
The following uncharacterized groups are probably not very 
distinct from Requienia. 
roucastA, Munier-Chalmas, 1873. 7. carinata, Matheron. 
Urgonian. 
MATHERONTA, Munier-Chalmas, 1873. JM. Virginie, Se. Ur- 
gonian. 
ETHRA, Matheron. £. Munieri, Math. Urgonian. 
Caprina, C. d’Orbigny, 1823. 
Etym.—Caprina, pertaining to a goat. 
Syn.—Spherucaprina, Gemm.  Plagioptychus, Matheron, 
1842. Gemmellaria, Munier-Chalmas. 
Distr.—Fossil, 10 sp. Upper Greensand and Lower Chalk ; 
Bohemia, France, Texas. 
Shell with dissimilar valves, with subspiral beaks; fixed 
valve conical, marked only by lines of growth and a ligamental 
groove ; hinge-margin with several deep cartilage-pits ; and one 
large and prominent tooth on the posterior side; free valve 
oblique or spiral, thick, perforated by one or more rows of 
flattened canals, radiating from the umbo and opening around 
the inner margin; anterior tooth supported by a plate which 
divides the umbonal cavity lengthwise, posterior tooth obscure ; 
hinge-margin much thickened, grooved for the cartilage. 
In C. adversa (exvii, 13; exviii, 19) the free valve is sinis- 
trally spiral; its cavity is partitioned off by numerous septa, 
and divided longitudinally by the dental plate. When young it 
is attached by the apex of the straight valve, but afterwards 
