ACEPUALA TESTACEA. 417 



We subsequently and gradually come to the Cythkre.e, Lam., 

 which have a fourth tooth on the right valve, projecting under the 

 lanula, and received into a corresponding cavity in the right one. 



Some of them have an elliptical and elongated lorm(l); others are 

 convex(2), and it is among these latter that we must place a cele- 

 brated species (Venus Bione, L., Chemn., VI, 27, 271), from whose 

 form originated the application of the name Venus to the genus. 

 Its transverse plates terminate behind in salient and pointed spines. 



There are some species of an orbicular form, and with slightly 

 hooked summits, in which the impression of the retractor of the 

 tubes forms a large and almost rectilinear triangle(3). 



When their animals are better known, we shall most probably 

 have to separate from the Cythereae, 



1. Those species of a compressed lenticular form, in which the 

 nates are united into a single point. The fold of the contour of the 

 mantle is wanting, and shows that their tubes are not protractile(4); 



2. Those of a convexly orbicular I'orm, in which the fold is not 

 only wanting, but where, as in the Lucinse, the impression of the 

 anterior muscle is very long(5)j 



3. The thick species with radiated ribs, in which the fold is 

 also wanting, and which connect the genus Venus with that of the 

 Venericardia(d). In the 



Capsa, Brug. 



Already separated from the former, there are two teeth on the 

 hinge on one side, and a single, but bifid one on the other; the lunula 

 is wanting, the shell convex and the fold, indicative of the retrac- 

 tor of the foot, considerable(7). 



Petricola, Lam. 



Also separated from the same genus; the Petricolse, on each side, 

 have two or three very distinct teeth on the hinge, one of which is 

 forked. The shell is more or less cordiform, but as they inhabit 

 the interior of stones, it sometimes becomes very irregular. Judg- 



(1) Venus gigantea, Encyc, 28, 3; — Ven. chione, Chemn., VI, 32, 343; — Ven. 

 erycina, lb., 347; — Ven. maculata, lb., 33, 345. 



(2) Ven. meretrix; — Ven. lusoria; — Ven. castrensis. 



(3) Venus exoleta, Chemn., VII, 38, 404 — the genus Ohdicdlus, Megerle. 



(4) Ven. smptu, Chemn., VII, 40,422. 



(5) Ven. tigrina, Chemn., VII, 37, 390; — Ven. punctata, lb. 397. 



(6) Ven. pectinala, Chemn., VII, 39, 419 — the genus Arthkmis, Oken. 



(7) Ven. deflorata, Chemn., IV, ix, 79—82. 



Vol. II.— 3 C 



