116 CLASS ACEPHALA. 
This character is particularly well marked in the oblong and 
slightly convex species. (Venus litterata, Chemn.) 
Some of them (the ASTART#, Sowerb., or CRASSINE, Lam.) 
have only two diverging teeth on the hinge, and approach the 
crassatelle in their thickness and some other characters. 
(Venus Scotica, &c.) 
Among the heart-shaped species, that is, those which are 
shorter, and have more convex nates, with more closely ap- 
proximated teeth, we should remark those where the plates 
or transverse striz terminate behind in crests or tuberosities, 
and those that have longitudinal ribs and crests elevated be- 
hind. (Venus dysera, Chemn.; Venus puerpera.) 
We subsequently and gradually come to the CYTHEREA, 
Lam., which have a fourth tooth on the right valve, project- 
ing under the /wnula, and received into a corresponding cavity 
in the right one. 
Some of them have an elliptical and elongated form: 
Venus gigantea, &c. 
Others are convex: Ven. meretria, &c. 
Among these we must place a celebrated species (Venus 
Dione, 1..), from whose form originated the application of the 
name Venus to the genus. Its transverse plates terminate be- 
hind in salient and pointed spines. 
There are some species of an orbicular form, and with 
slightly hooked summits, in which the impression of the re- 
tractor of the tubes forms a large and almost rectilinear tri- 
angle. (Venus exoleta.) 
When their animals are better known, we shall most pro- 
bably have to separate from cytherea— 
1. Those species of a compressed lenticular form, in which 
the nates are united into a single point; the fold of the con- 
tour of the mantle is wanting, and shows that their tubes are 
not protractile. (Venus scripta.) 
Cy = 
