Professor Owen, from an examination of Mr. Cuming's specimen, has given an account of the anatomy of this mollusk. (' Zool. Trans.,' vol. i.) He found the following to be the relative position of the animal :—The month turned towards the closed end of the chamber, which is consequently the anterior part. The heart and rectum near the side where the valves are connected by the ligament, or the dorsal part. The visceral mass projecting towards the opposite or neutral side. The siphon extending into the commencement of the calcare ous tube, which leads out of the anal or posterior part of the chamber. The fixed valve, which covers the rough surface of the porous rock or coral, like the tiling of a chamber-floor, and affords a smooth polished surface for the support and attachment of the animal, is the left valve : the right valve remains free, or is connected only to the soft parts and cardinal ligament, in order to assist in the excavating and respiratory actions.
The shelly substance of the fixed valve passes without interruption into that of the tube ; a slight ridge circumscuibing the entry of the tube into the chamber indicating the line of separation, unless the extent of the valve be limited to that of the internal nacreous deposi tion. The tube of an oval form, 7 lines by 5 in diameter. The cal careous walls Lth of an inch in thickness at the outlet, and about Lth at the opposite extremity. The free valve unequally triangular, with the angles rounded off, about the thickness of a sixpence, moderately concave towards the soft parts, and striated only in the direction of the layers of increment on the outer surface, as in most of the Pylori dean Bivalves of M. de Blaiuville. The layers gradually increase towards the dorsal edge for a little more than one half of the valve, beyond which the layers continue of almost equal breadth. " This growth of the valve," adds Mr. Owen, " corresponds to the direction in which the chamber is enlarged, which is principally on the dorsal, dextral, and anterior aides :. now this is the mode of enlargement best adapted for the full development of the ovary ; so that it would seem that the Claragella continues for a time to work its way into the rock without material increase of size, leaving behind it a calcareous tube, which marks its track ; after which it becomes stationary, and limits its operations to enlarging its chamber to the extent necessary for the accomplishment of the great object of its existence." The mantle enveloping the body is like a shut sac, but perforated for the siphon and foot, the opening for the latter being reduced to a small slit. M. Riippell observed an analogous orifice in the corre sponding part in Aspergillum, namely, that which is next the sunken sieve-like extremity of the tube, and by which he supposes the water necessary for respiration to be received when the retreating tide leaves exposed the expanded siphonic extremity. Professor Owen is of opinion that this cannot be its use in those species of Claragella which exist at depths too great to allow of their being ever left with the siphonic aperture out of water ; but that it must serve to keep up a communication with the neighbouring cavities of the rock, i by means of the calcareous tubules, the formation of which is deter mined by the proximity of these cavities. When therefore the
Claragella, by a sudden contraction of the adductor muscles, has forcibly expelled the brauchial currents from the siphon, as was observed by Mr. Stutchbury, the space between the free valve and the walls of the chamber would be simultaneously filled, either by water rushing in through the tubules, or forced out from the branchial cavity through the small anterior orifice of the mantle. To assist this operation there is a proportional development of the muscular system, which is remarkably powerful. The impression of the great or posterior adductor is carried two lines beneath the surface of the chamber posteriorly, but gradually rises to the level.of the valve. The impression of the smaller anterior adductor is more faint, and is continued into the sinuous pallial impression, which follows the contour of the anterior margin of the valve at about two lines' distance from it. In the fret valve the last two muscular impressions are separate. The outer dermoid layer of the mantle is extremely thin, and, where it does not line the valves, is mottled with minute dark spots, less numerous than those on the skin of Cephalopods, and presenting, under the microscope, a glandular appearance. The muscular layer, after forming the siphon and its retractors, is confined to the anterior part of the mantle, where it swells into a thick convex' mass of interlaced and chiefly transverse fibres, and forming, Professor Owen supposes, one of the principal instruments in the work of excavation. No fibres could he detected in other parts of the mantle ; nor could any be expected in a mantle which had no lobes to be retracted. The siphon, in the contracted state, formed a slightly-compressed cylindrical tube, half an inch in length, and the same in the long diameter, traversed longitudinally by the branchial and anal canals, separated from each other by a muscular septum, extending to the end of the siphon, beyond which tho two tubes do not separately extend outwards, agreeing iu this respect with Gastrochama and Aspergillum. Muscular walls of the siphon two lines in thickness; the septum separating the bronchial and anal canals one line ; diameter of each canal about one line ; inner extremity both of the anal and respiratory tube provided with a valvular fold: termination,' beset with short papal:ie. The retractor muscles attach the siphon to the posterior adductor on one side, and to the anterior extremity of the oval mass of muscular fibres above mentioned on the other, leaving an intermediate apace on both sides the body, which exposes part of the gills and labial tentacles. The muscular mass which bounds the anterior part of the animal's body in oval, 000 inch three lines long, eight lines broad, and varying iu thickness from two to three lines: it is smooth and convex externally, and hollowed out within to lodge the viscera at the base of the foot, for the passage of which it leaves the small orifice above mentioned. The margina attached to the valves are more or less irregular : that affixed to the loose valve is the broadest, being at the ventral extremity three lines in length.