100 PARTS OF BIVALVE SHELLS. 



modified to form the joint or hinge upon which the two valves 

 open and shut. This part is called the " cardinal edge" (jig. 

 119), and generally presents certain prominences and depressions ; 

 the projections of one valve interlocking with the depressions of 

 the other. The projections, or " teeth," together with the cavi- 

 ties, or " cardinal pits" (p), are very regular in their formation 

 in each genus and species of bivalve, but what is of more impor- 

 tance is, that every modification in the structure of the hinge is 

 generally found to coincide with some recognisable and more or 

 less important difference in the organization of the soft parts ; so 

 that conchologists have justly attached great value to the charac- 

 ters derivable from the hinge, especially for the purpose of gene- 

 ric distinctions. When the teeth are situate beneath the apex (s) 

 or centre of the hinge, they are called cardinal, or primary / 

 when they are removed from the centre of the hinge, they are 

 named lateral teeth ; when two only are present, one is called 

 anterior, the other posterior ; when there are three, they are dis- 

 tinguished respectively as anterior, median or middle, and pos- 

 terior teeth ; but when the hinge is composed of a great number 

 of teeth, it is said to be " serial," as in Area (Jig. 97) ; some 

 hinges have no visible teeth, and are termed inarticulate. 



The direct medium of union of the two valves is a dense fasci- 

 culus (bundle) of elastic fibres, generally of a brown colour, called 

 ligament, or elastic ligament. The fibres of this part are at- 

 tached by their extremities to the two valves, which, in most cases, 

 present a particular depression for their reception. The ligament 

 is always so long as to prevent the actual closing of the valves, 

 except when its elasticity is overcome by a certain force, as by 

 that of the contraction of the adductor muscle or muscles : thus, 

 the inorganic power of elasticity is made the direct antagonist of 

 a vital and muscular contraction ; and as the open or expanded 

 condition of the bivalve shell is that which the exigences of the 

 animal most constantly require, it is assigned to a force which 

 can act without ever causing fatigue, while the occasional or 

 protective action of forcibly closing the valves, is due to an ac- 

 tion under the immediate control of the will or instinctive sensa- 

 tion. The modifications of the internal surface of a bivalve 

 shell are caused by the structure of the animal inhabiting it : 

 hence, they afford the characters by which the habits of. an ex- 

 tinct genus may be to a great extent determined. 



These modifications are marked on the last figure. 



The pallial impression shows the development of the muscu- 

 lar margin of the mantle. 



