ACEPHALA TESTACEA. 403 



serted into the extremities or the valves, which serve to close them. 

 In the 



Arca, Lam., 



Or the Arcae 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 centre, because 

 there is a horny plate or tendinous fillet before the abdomen of the 

 animal(l) that serves for a foot, and by which it adheres to sub- 

 merged bodies. They are found in rocky bottoms near the shore, 

 and are usually covered with a haii-y epidermis. They are not much 

 esteemed for the table. 



Some species are found in the Mediterranean(2), and a great 

 many fossil, in strata anterior to chalk, particularly in Italy. 



Certain Arcfe 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^a(3). 



We ought also, it is probable, to separate the species with well 

 marked ribs, and completely closing and interlocking edgesj for we 

 must presume that their animal is not fixed, but rather resembles 

 that of a Pectunculus(4). 



We have a still better warrant for removing the Area tortuosa, 

 Chemn., VIII, liii, 524, 525, in its fantastic figure and unequally 

 oblique valves(5). 



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(6). 



(1) The Daphne, Poll. 



(2) ^rca JVox, Chemn., VII, liii, 529, 5Sl;jlrca harbata. Id., liv, 535, 537; 

 A. ovata, lb., 538; A. magellanica, lb. 539; A. reticulata, lb. 540; A. Candida, 

 Id., Iv, 542, 544; A. indica, lb., 543; A. cancellata, Schrosd., Intr., Ill, ix, 2. 



(3) Arca cucullata, Chemn., VII, liii, 526, 528; Cuculkea crassatina. Lam., 

 Ann. du Mas., VI, 338. 



(4) Arca antiquata, L. Chemn., VII, Iv, 548, 549; A. senilis. Id., Ivi, 554, 556; 

 A.granosa, lb. 557; A. corhiculata, lb., 558, 559; A. rhombo'i'dea, lb., 553; 

 A. jamaicensis. List., 229, 64. 



(5) It forms the genus Trisis, Oken. 



(6) Arca pilosa, L., Chemn., VII, Ivii, 565, 566; Arc. glycimeris, lb., 564; 

 A. decussata, lb., 561; i. sequilatera, lb., 562; A. undata, lb., 560; A. marmo' 

 rata, lb., 563; A, pectunculus, Id., Iviii, 568, 569; A. pcctinata, lb., 570, 571- 



