ACEPIIALA TESTACEA. J 7 7 



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 Tridaaus, Brug., have a shell 

 greatly elongated transversely, aud equivalvc ; 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 like most oil. 

 but its organs arc 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 

 Bide there is a third smaller opening, corresponding with the anus, so that there is no need of a pas 

 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 

 iniddie of the margin of the valves. 



In the Tridacna of Lamarck the shell lias 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 yii/as, 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 Lainellebranchiate 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.] 



Hippopm, Lam. — The shell is closed and flattened in front, as if it had been truncated. [//. maeulatus, from 

 the Sjuth Seas, is the only species.] 



Cham, i. Brag. — 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. < un n also their interior 

 cavity has this form, though nothing on the exterior surface may indicate it. The animal (JPrilopitt, l'olii has a 

 t 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 Cleidothcerui, Stutchbury, has a very exact resemblance to Chama, 

 but is worthy generic distinction from the remarkable circumstance of its internal hinge cartilage having an 

 [ungated testaceous appendage, in form resembling the human clavicle. The only species is from Port Jackson.] 



The Dieeratet, 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. 

 Onlj known in a fossil state.] t 



Itocardia, Lam., has a free, regular, ventricose shell, the beaks of the valves distant, turned backwards, and 

 involute. The animal (Glotltu, Poli) differs from that of Chama only in having a larger and oval foot, and in the 

 ■interior aperture of the cloak beginning to assume the ordinary proportion. Oiie species (Chama cor, Linn.) is 

 found in the Mediterranean [and German Ocean]. 



THE FOURTH FAMILY OF THE ACEPIIALA TESTACEA,— 



Tiik 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 

 a always an adductor muscle at each extremity, and a foot, which in general enables the animal to 

 p, We ma\ 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 he traced on the shell by the greater or less 

 depth of marks made by t lie insertion of the edges of the cloak previous to its uniting with the impres- 

 sion of the posterior transverse muscle. 



Tiik Cocki.es {('ardiain, Linn.) — 

 Have, like most other Bivalves, a shell with equal ventricose valves, with prominenl beaks carved 

 towards the binge, which gives them, when we view them laterally, the figure of a heart, whence their 

 Ribs, more ot less prominent, trend from the beaks to the margins of the \,\\\<^. Bat 

 thai which distinguishes the I ardia is thi ir hinge, where we may notice, on both rides in the middle, 

 two little teeth ; and at some distance before and behind, a tooth or prominent lamina. The animal 

 r, Poli h ti usually in ample aperture in the cloak, a verj large foot, bent in the middle, with 

 it- point directed forwards, and two short or but moderate!] long tol 



