ACEPHALA TESTACEA. 
377 
The family comprises only the genus 
Chama, Linn., — 
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 justly been subdivided. The Tridacncn, Brug., have a shell 
greatly elongated transversely, and equivalve ; the superior angle, which answers to tlie head and 
summit, very obtuse. The animal is very remarkable, for it is not placed in the shell like most others, 
but its organs are all directed, or as it were pressed out, forwards. There is a wide opening in the 
anterior side of the cloak for the passage of the byssus : a little beneath the anterior angle there is 
another aperture by which the water gets access to the branchiae ; and in the middle of the inferior 
side thei-e is a third smaller opening, corresponding with the anus, so that there is no need of a passage 
in the posterior angle, which is solely occupied by a cavity of the cloak, open only to the third aper- 
ture, which has been just mentioned. There is but a single transverse muscle, corresponding to the 
middle of the margin of the valves. 
In the Tridacna of Lamarck the shell has in front, like the cloak, a large aperture with denticulated margins 
for the [exit of the] byssus, which is distinctly tendinous, and continuous with the muscular fibres. Such is the 
Chama gigas, Linn,, of the Indian Ocean, famous for its enormous size. There are individuals which weigh more 
than three hundred pounds. The tendinous byssus by which it is suspended to rocks is so large and tough as to 
require to be cut with an axe. The animal is edible, although very hard. [It is placed in the shell somewhat 
differently from other Lamellebranchiate Mollusca ; for, from a peculiar inversion, it is found that its different 
parts have not their ordinary correspondency, — a circumstance which Blainville thinks is owing to the suspended 
condition of the shell.] 
Hippopus, Lam. — The shell is closed and flattened in front, as if it had been truncated. [H. macrdatus, from 
the South Seas, is the only species,] 
Chama, Brug. — Shell irregular, inequivalved, often lamellated and spinous, and attached to rocks, corals, &c., 
in the manner of Oysters. The summits are often very protuberant, unequal, and curled. Often also their interior 
cavity has this form, though nothing on the exterior surface may indicate it. The animal (Psilopus, Poll) has a 
small foot, bent almost like that of a man. The tubes, if there are any, are short and separate, and the aperture 
through which the foot passes is little larger than them. There are some living species in the Mediterranean ; 
and there are also several fossil species. [The Cleidotluerus, Stutchbury, has a very exact resemblance to Chama, 
but is worthy generic distinction from the remarkable circumstance of its internal hinge cartilage having an 
elongated testaceous appendage, in form resembling the human clavicle. The only species is from Port Jackson.] 
The Dicerates, Lam., do not appear to differ from Chama in anything essential ; but their hinge tooth is very 
thick, and the spirals of their valves are so prominent as to prompt a comparison of their form with two horns. 
[Only known in a fossil state.] 
Isocardia, Lam., has a free, regular, ventricose shell, the beaks of the valves distant, turned backwards, and 
involute. The animal {Glossus, Poll) differs from that of Chama only in having a larger and oval foot, and in the 
anterior aperture of the cloak beginning to assume the ordinary proportion. One species {Chama cor, Linn.) is 
found in the Mediterranean [and German Ocean]. 
THE FOURTH FAMILY OF THE ACEPHALA TESTACEA,— 
The Cardiacea, — 
Have the cloak open in front ; and there are besides two separate apertures, (one for respiration and 
one for a vent,) which are prolonged in tubes, sometimes distinct, and at others united together. There 
is always an adductor muscle at each extremity, and a foot, which in general enables the animal to 
creep. We may regard it as a very general rule, that those which have long tubes live buried in the 
mud or sand. This peculiarity of their organization is to be traced on the shell by the greater or less 
depth of marks made by the insertion of the edges of the cloak previous to its uniting with the impres- 
sion of the posterior transverse muscle. 
The Cockles {Cardimn, Linn.) — 
Have, like most other Bivalves, a shell with equal ventricose valves, with prominent beaks curved 
towards the hinge, which gives them, when we view them laterally, the figure of a heart, whence their 
generic name. Ribs, more or less prominent, trend from the beaks to the margins of the valves. But 
that which distinguishes the Cardia is their hinge, where we may notice, on both sides in the middle, 
two little teeth ; and at some distance before and behind, a tooth or prominent lamina. The animal 
{Cerastes, Poli) has usually an ample aperture in the cloak, a very large foot, bent in the middle, with 
its point directed forwards, and two short or but moderately long tubes. 
