Okder 1. ACEPHALA TESTACEA. - 369 



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 Tridacrue, Brug., have a shell 

 greatly elongated transversely, and equivalve ; the superior angle, which answers to the head and 

 summit, very obtuse. The animal is very remarkable, for it is not placed in the shell hke 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 branchire ; and in the middle of the inferior 

 side there 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 

 Clmma gigas, Linn., of the Inxlian 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 difterent 

 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. maculatus, 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 {Psilopiis, 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 JVlediterranean ; 

 and there are also several fossil species. [The Cleidot/icerus, 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 Fort 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) difl'ers 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. Out 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 aud 

 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 (Cardium, 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, Poll) 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. 



