ACEPHALA TESTACEA. 409 



Judging from the impressions of its muscles on them, their animal 

 must resemble that of the Carditae and Unios(l). 



Both of them approach the Cardia in their general form and the 

 direction of their ribs, I suspect that this is also the place for the 



Crassatella, Lam., — Paphia, Roiss., 



Which has sometimes been approximated to Mactra, and at others 

 to Venus; the hinge has two slightly marked lateral teeth, and two 

 very strong middle ones, behind which, extending to both sides, is a 

 triangular cavity for an internal ligament. The valves become 

 very thick by age, and the impression made by the margin of the 

 mantle leads to the belief that there are no protractile tubes(2). 



FAMILY IIL 



CHAMACEA. 



The mantle closed and perforated by three holes, through 

 one of which passes the foot ; the second furnishes an entrance 

 and exit to the water requisite for respiration, and the third 

 for the excretion of fasces : these two latter are not prolonged 

 into tubes as in the subsequent family. It only comprises the 

 genus 



Cham A, Lin,, 



Where the hinge is very analogous to that of a Unio, that is to say, 

 the left valve near the summit is provided with a tooth, and further 

 back with a salient plate, which are received into corresponding 

 fossae of the right valve. This genus has necessarily been divided 

 into the 



Tridacna, Brug., 

 The shell greatly elongated transversely, and equivalve; the su- 



(1) Fenws mJncate, Chemn., V], XXX, 314,315, and the fossil species, Lam., 

 Ann. du Mus., VII and IX, pi. xxxi and xxxii. 



(2) Venus ponderosa, Chemn., VII, Ixix, A — D, or Crassatella tumida, Lam., 

 Ann. du Mus., VI, 408, 1; perhaps the Mactra cygnus, Chemn., VI, xxi, 207; — 

 Venus divaricata, Chemn., VI, xxx, 317 — 319. This genus also comprises many 

 fossil species, particularly abundant near Paris. See the work of M. Deshayes, 



Vol. II.— 3 B 



