92 
MOLLUSCA. 
serted into the extremities of the valves, which serve to close them. 
In the 
Arca, Lam., 
Or the Arcse properly so called, the hinge is rectilinear, and the 
shell most elongated in a direction parallel to it. The summits are 
generally convex, and curve over the hinge, but are separated from 
each other. The valves do not close perfectly in the centi’e, because 
there is a horny plate or tendinous fillet, before the abdomen of the 
animal * * * § ** that serves for a foot, and by which it adheres to submerged 
bodies. They are found in rocky bottoms near the shore, and are 
usually covered with a hairy epidermis. They are not much esteemed 
for the table. 
Some species are found in the Mediterranean f , and a great 
many fossil, in strata anterior to chalk, particularly in Italy. 
Certain Arcee in which the teeth of the two ends of the hinge as- 
sume a longitudinal direction, are distinguished by Lamarck under 
the name of Cucull^ea J. 
We ought also, it is probable, to separate the species with well 
marked ribs, and completely closing and interlocking edges ; for we 
must presume that their animal is not fixed, but rather resembles that 
of a Pectunculus §. 
We have a still better warrant for removing the Arca tortuosa, 
Chemn., VIII, liii, 524, 525, in its fantastic figure and unequally obli- 
que valves ||. 
Pectunculus, Lam. 
The hinge forming a curved line, and the shell lenticular; the valves 
always close completely, and their summits are approximated. The 
animal, Aximea, Poll, is furnished with a large compressed foot with 
a double inferior margin which enables it to crawl. They live in 
ooze. Some species are found on the coast of France 
Nucula, Lam. 
The Nuculae are Arcae, in which the teeth are arranged on a broken 
line. Their form is elongated, and narrowed near the posterior ex- 
tremity. Their animal is unknown, but is probably not far removed 
from those of the preceding shells 
This has long been the place assigned to the 
* The Daphne, Poli. 
-j- Arca Noce, Chemn., VII, liii, 529, 531 ; — Arca harbata, Id., liv, 535, 537 ; — 
A. ovafa, Ib., 538 ; — A. magellanica, Ib., 539; — A. reticulata, Ib. 540; — A. Candi- 
da, Id., Iv, 542, 544 ; — A. indica, Ib., 543 ; — A. canceUata, Schroed., Intr., Ill, 
ix, 2. 
X Area cucullata, Chemn., VII, liii, 526, 528 ; — Cucullcca crassatina, Lam., Ann. 
du Mus., VI, 338. 
§ Arca antiquata, L. Chemn., VII, Iv, 548, 549 ; — A. senilis, Id., Ivi, 554, 556 ; 
— A. granosa, Ib., 557 ; — A. corbicidata, Ib., 558, 559 ; — A. rhombuidea, Ib., 553 ; — 
A. janiaiccnsis. List., 229, 64. 
II It forms the genus Trisis, Oken. 
^ Arca x>ilosa, L., Chemn., VII, Ivii, 565, 566 ; — Arc. glycimeris, Ib.. 564 ; — A. 
deCKSSata, Ib., 561 ; — A. cequilatera. Id., 562 ; — A. undata, Ib., 560 ; — A. marmorutu, 
Ib., 563 ; — A. pectunculus. Id., Iviii, 568, 569 ; — A. pectinata, Ib., 570, 571. 
** Arca pellucida, Chemn., VH, liv, 541 ; — Arca rostrata, L., Id., Iv, 550, 551 ; 
— Arc. pelta, Ib., 546 ; — Arc. nucleus. Id., Iviii, 574. 
