708 



CONCHIFERA. 



the shell, of various length and thickness, upon 

 which the hinge occurs, is called its cardinal 

 edge. In the hinge two structures are appa- 

 rent : 1st, an elastic ligament, the position of 

 which is variable ; 2d, projections and corres- 

 ponding cavities on either valve, destined un- 

 doubtedly to give additional strength to their 

 union. 



1. The ligament. The ligaments of bivalve 

 shells are distinguished into two kinds, accord- 

 ing to their structure and their position: they 

 are internal when they are completely hidden 

 by the cardinal edge of the shell ; they are 

 external when they appear on the outside be- 

 yond this edge. The internal ligament is com- 

 posed of a great number of highly elastic 

 fibres, parallel to one another, and perpendi- 

 cular to the valves they connect. They are 

 secreted by a lamina of the mantle, projecting 

 upon the back of the animal, and penetrating 

 between the edges of the two shells. The 

 fibres of the ligament secreted when the shell 

 is partially open, are of too great length when 

 it is shut, so that when the valves are ap- 

 proximated to one another these fibres are 

 forcibly compressed, and their elasticity is 

 brought into play, by which it is only necessary 

 for the animal to relax its adductor muscles in 

 order to have the fibres of the ligament, in 

 their effort to regain their natural length, force 

 the valves apart from one another to a deter- 

 minate extent. When the ligament is external, 

 it rests upon the prominent parts of the cardinal 

 edge, parts to which the title of ni/wplue. has 

 been given (a, a,fg. 365). When the ligament 

 is of this kind, it consists of two distinct 

 layers, one external, thin, and very strong, 

 composed of transverse fibres, which extend 

 from one nympha to the other, and are strongly 

 inserted within a groove hollowed out of the 

 base of each of them. The other portion of 

 the external ligament is of precisely the same 

 structure as that of internal ligiments, and is 

 comprised between the nymphae and the outer 

 layer, of which we have- just made mention. 

 The action of this ligament is also precisely 

 the same : it forces the valves apart when the 

 animal ceases to maintain its adductor muscles 

 in a state of contraction. 



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 nympha' 

 more prominent externally than the cardinal 

 edges, hut contracting gradually under this 

 edge in proportion as the nymphae 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 tiie 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 nymphse 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 : Soldi, 

 Panopus, Thracia, Calcine/la, Amphidesma , 

 Liitraria, Mactru, Mi/a, Crassatclla. 



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 Area family. The 

 superior surface of the hooks in these genera 

 (Area, Pcctuncultis, Cucullaa) 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 (Pernu, Crenatida), 

 instead of having a single ligament, have a 

 regular series of fossicuhe, 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 hingn 

 is said to lie 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 

 genus denominated Pliolus. In the interior of 

 the valves of this genus there always exist 

 two kinds of large curved processes, extend- 

 ing from the interior summit of the hooks 

 ("> fg- 364), and advancing nearly to the 

 middle of the valves. According to our views 



