ACEPIIALA TESTACEA. 
97 
to Venus ; the hinge has two slightly marked lateral teeth, and two 
very strong middle ones, behind Avhich, 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*. 
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 Avater requisite for respiration, and the third for the 
excretion of faeces ; these two latter are not prolonged into tubes as 
in the subsequent family. It only comprises the genus 
Chama, Lm., 
Where the hinge is very analogous to that of a Unio, that is to say, 
the left valve near the summit is provided Avith a tooth, and further 
back AAuth a salient plate, Avhich are received into corresponding 
fossae of the right A’^ah^e, This genus has necessarily been di\dded 
into the 
Tridacna, Brug,, 
The shell greatly elongated transversely, and equiA'alve ; the supe- 
rior angle, Avhich ansAvers 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 Avere, pressed 
out before. The anterior side of the mantle is Avidely opened for 
the passage of the byssus; a little beloAV the anterior angle is another 
opening AA^hich transmits Avater to the branchiae, and in the middle of 
the inferior side is a third and smaller one AA'hich 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 Avhich Ave have just spoken. 
There is but a single transverse muscle, corresponding to the 
middle of the margin of the A^alves. In 
Tridacna, Lam., 
Or the Tridacnae properly so called, the front of the shell as Avell 
as of the m intle has a Avide opening Avith notched edges for the trans- 
mission of the byssus, Avhich latter is CAudently tendinous, and con- 
tinues uninterruptedly Avith the muscular fibres. 
* Venus ponderosa, Chemn., VII, Ixix, A — D, or Crassatella tumidu, Lain., Ann. 
du Muss., VI, 408. 1 ; perhaps the Mactra epynus, Cheiun., 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. Deshayew. 
VOL. III. H 
