The structure of the valves gives sufficient indications of the manner in which they arc formed. M. Dufresne, in a paper published in the ?inales du Museum 1, p. 465, advanced the singular opinion, that the animals quitted their old shells when they became too small, and loaned new ones suited to their size. The arguments by which it is supported indicate an ignorance of the structure of the shell, and the relations of the parts of which it consists. To us it appears plain, that each valve is increased in two directions, the elevated part by an extension of the tubuli at the base, and the depress ed part by the application of fresh matter to the side. The stria, which arc the indications of successive de positions of matter, and the structure of the valves them selves, point out this mode of enlargement as the only one which can take place, even on the supposition that the shell is frequently renewed. By the growth of the elevated parts, the shell increases in height and diameter at the base, while the growth of the depressed parts preserves to the mouth suitable dimensions for the cor respnnding increase of size in the parts of the operculum. It is obvious that this increase of diameter at the base must be accompanied by a corresponding enlargement of its covering. This takes place by the extension of the horizontal tubuli, and each enlargement is marked by a concentric ridge.
This opinion which we have espoused, and which we find hinted at by Lamarck, in his " Histoire Araturelle des At:in:aux sans Vertebres," v. p. 398, is founded oh the structure of the different valves, the indications of the layers of growth, and the manner in which the valves are separable from each other and from the base. 'We might have entertained some suspicions of error, had we not attended to the morbid appearances of the shell, the restraints imposed on its growth by the situation in which it lives, but especially the manner in which frac tures are healed, and abstracted parts restored by the secretion of new matter. Such observations have re moved all suspicion, and demonstrated the truth of the explanation offered. It may be added, however, that, in the case of the inversely conical shells, the increase pro bably takes place at the mouth.
The mouth of the shell is closed by the cloak of the - animal, leaving in the centre a tubular or linear aper ture for the protrusion of the feet and entrance of the water. This part cf the cloak is protected by testace ous plates, which, by their union, form a lid to the mouth of the shell, for the protection of the contained inhabitant. The valves of the lid are four in number,
two on each side the mesial line, or orifice. In some genera the lateral valves are united. The operculum of this order may with propriety be compared to the shelly plates of the body of the preceding order—and the shelly body of thi's order corresponds to their pe duncle, circumstances indicated by the muscular at tachments of the animal. The continued action of the valves of the lid obviously assist in wearing down and enlarging the aperture of the shell.
There is little known, either with regard to the or gans of digestion or respiration, in the animals of this order. In their manner of reproduction, they appear to resemble those of the preceding order. Ellis " found the lower part of the shell, which contained a cavity equal to two-thirds of the whole, full of spawn." The genera of this order divide themselves into three fami lies, from circumstances connected with the shell.
1. Family.—The shell in this family consists of six valves, and the lateral valves of the lid are divided. It contains five. genera.
1, Tubicinella.—The form of the shell in this genus is inversely conical, the apex which constitutes the base being truncated. It consists of a series of horizontal rings, whichmark the successive periods of grow th, and there are six vertical grooves, which indicate the divi sions of the valves. The increase of the shell with age, in this genus, probably takes place by the addition of a new ring to the mouth. The testaceous plates of the lid are all of equal size. The inferior aperture of the shell is open, or simply closed by the integuments of the cloak. The animal resides in the skin of the whale, the lower rings being inserted in the fat, while one or more of the upper ones appear above the cuticle. The T. balxnarum is the only known species.
In the remaining genera of this family, and the others which follow, the shell is conical, its truncated apex be ing the mouth ; and its mote olgrowth such as is detail ed in the general remarks on the order.
2. Coronula. The base of the shell is open, as in the pre ceding genus. The valves of the lid are unequal in size, the dorsal ones being small. The animals included un der this genus likewise inhabit the skin of the whale. Several species of this genus are known. The C. dia dems holds a place in the British Fauna.
3. Chelonobia.—ln this genus the base of the shell is likewise open, but it differs from the preceding in the plates of the lid being all of equal size. The C. testa dinaria, a species which resides on turtles, is the type of the genus.