In the extensive series of Conchiferous mol lusks, some modifications, as might have been anticipated, are met with in the conformation of the ligament, external as well as internal. If many members of the family of the Dimy aria be examined, the ligament, very prominent outwards, will be seen bearing upon nympha3 more prominent externally than the cardinal edges, hut contracting gradually under this edge in proportion as the nymphm become shorter, until in some species we find that, still preserving the structure of the external ligament, the whole of this apparatus is never theless entirely hidden under the superior edge of the shell. This point attained, the external ligament alters by insensible degrees into a ligament completely internal ; that is to say, the exterior fibrous layer diminishes gradually, and at length disappears entirely when the ligament is much developed upon certain in ternal parts of the hinge. Our own opinion is, that the ligament is internal when the nymphae, having undergone certain modifications, have been transferred to the interior, and have as sumed the form of acetabula. The ligament is sub-internal when the nympha, of less depth, still show a portion of the ligament externally; finally the ligament is external when the nympliw are situated towards the upper edge of the shell. This displacement of the ligament, and of the solid part which gives it insertion, is very well seen in the succession of the following genera : Solen, Panopus, Thracia, Amphidesma, Lutraria, .11flactra, Crassatella.
In those shells in which the beaks or hooks are of great size, and spirally turned to one side, the ligament, in keeping pace with the growth of the covering, bifurcates at its anterior part, and this bifurcated part then becomes useless. This circumstance is particularly remarked in the Isocardium and the Chama. The ligament also presents a very remarkable peculiarity in the three genera of the Arca family. The superior surface of the hooks in these genera ( Area, Pectunculus, Cucullwa) is of greater or less breadth, flattened, triangular, some times furrowed, and has a thin ligament, re sembling an elastic web, strongly attached to it..
The ligament in the greater number of the genera of the Monomyaria is situated within a triangular groove or depression of a breadth corresponding to its dimensions. In one fa mily, that, namely, of the Malleacea of La marck, several genera ( Perna, Crenatul a ), instead of having a single ligament, have a regular series of fossicula2, in each of which a ligament is implanted.
Cardinal edge.—The cardinal edge presents a great number of modifications. Sometimes it is simple, and of various degrees of thick ness, in which case the hinge is said not to be articulated ; sometimes it presents projections and reciprocal cavities, in which case the hingo is said to be toothed or articulated upon the' cardinal edge. These projections and hollows are remarkably regular in their formation, and every change in their appearance commonly coincides with one of greater moment in the organization of the animal. This remarkable coincidence, to which only a very few exceptions are yet known, has led conchologists to attach great value to the characters derivable from the hinge, and Lamarck, among others, has grouped several families and a great number of genera after them. We believe, with this celebrated naturalist, that the hinge supplies excellent characters for the distinction both of families and genera, but we have been led to this con clusion by viewing the subject in a different point of view from that taken by Lamarck.
Every conchologist knows the interesting 0-enus denominated Pholas. ln the interior of tbe valves of this genus there always exist two kinds of large curved processes, extend ing from the interior summit of the hooks (a, jig. 364), and advancing nearly to the middle of the valves. According to our views these appendages are the first parts of the cardinal teeth. There. is one fact which deserves to be insisted on in connexion with this genus ; it is that there are no ligaments found, and that the cardinal edge, folded in _ _ - 1 upon itsen TC11 Ire Slt7' G111 7thne), is not flattened and placed in the same manner as in the other conchifera. Another .
on the other for their reception mutually. The depressions are called cardinal pits. These cavities and these projections present a great variety of modifications which cannot be well understood without a long and careful study of the conchiferous tribes generally. When the teeth are collected under the hook, they pre serve the title of cardinal (b, fig. 365) ; when circumstance or equal nnpor tance to be mentioned is that the processes, of which we have just spoken, are buried in the substance of the animal, and covered with a duplicature of the mantle which accompanies them as they plunge amid the visceral mass. Without leav ing the genus Pholas, the cuil lerons may be seen gradually contracting in their breadths, be coming shorter, and approaching nearer and nearer to the edge.
But if other shells be examined, which obviously form the links of transition from the Pholada to the Sasicava, or Petricola, the processes are found to turn upon the edge, to become coherent with it so as to form a salient margin, and by their free extremity to produce a projection (b, fig. 364). In our opinion the toothings of the hinge of all the other bivalve shells are produced in the same manner; but with such modifications as rarely admit of those relations being traced which are to our mind obvious in those genera that have just been particularly mentioned. With regard to the shells of the genera in which the hinge is complicated, of which the cardinal edge is thickened, and the cavity of the hook partly tilled by the external layer of the shell, it is difficult to imagine in what manner the suc cessive growth of the hinge has taken place, and to make out its analog,y in point of struc ture with that of the Petricola pholadiformis and of the Phobida generally. To discover this it is necessary to break a great number of the shells, or to make various sections of the edge, when the direction of the denticulations xvith which it is furnished must be followed. The teeth of the hinge will then be seen arising froin the summit of the hook (c, 364), becoming developed, and forming a solid arc, surrounded and hidden by the matter of the cardinal edge itself, and these arcs thus disen gaged will be found to present the strongest analog-y with those of the Pholada. It is from viewing the hinge in this manner that we have been induced to think that its structure was in reality of sufficient importance to make it be constantly appealed to for the distinguishing characters of genera.