ACKPIIALA TBSTACEA. 97 



tu Venus ; th- hinge has two slightly marked lateral teeth, and two 

 very strong middle ones, behind which, extruding to I oth 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 tliat there are no protractile tubes*. 



FAMILY III. 



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 



ivtion of faeces; these two latter are not prolonged into tubes as 



in the subsequent family. It only comprises the genus 



CHAMA, 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 supe- 

 rior angle, which answers to the head and summit, very obtuse. 



The animal is very singular, inasmuch as it is not, like most of 

 the others, placed in the shell, but is directed, or, as it were, pressed 

 out before. The anterior side of the mantle is widely opened for 

 the passage of the byssus; a little below the anterior angle is another 

 opening which transmits water to the branchiae, and in the middle of 

 the inferior side is a third and smaller one which corresponds to 

 the anus, so that the posterior angle transmits nothing, and is 

 only occupied by a cavity of the mantle open at the third orifice, 

 of which we have just spoken. 



There is but a single transverse muscle, corresponding to the 

 middle of the margin of the valves. In 



TRIDACNA, Lam., 



Or the Tridacnac properly so called, the front of the shell as well 

 as of the in <ntle has a wide opening with notched edges for the trans- 

 mission of the byssus, which latter is evidently tendinous, and con- 

 tinues uninterruptedly with the muscular fibres. 



* Ven\a pondtrosa, Chemn., VII, Uix, A D, or Crassatella tumuta, Lam., Ann. 

 hi MUX-., \ I. 4 OS. 1.; perhaps the Mactra cygnus, Chemn., VI, xxi, 207 ; Vanu 

 i!u-nrti;t/,i, Clumn., VI, xxx, 317 319. This genus also comprises many fossil 

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



VOL. in. H 



