BIVALVE SHELLS. 465 



vided into two parts, either by a line or by an alteration in 

 the character of its markings, or by a change of its colour. 



The Hinge. The hinge consists of the (1) ligament, and 

 (2) of certain projections and fossulae in the cardinal margin 

 of the shell, those of one valve fitting into the corresponding 

 parts of the other. In a very few instances the teeth are op- 

 posite ; and in a few other instances the tooth has no opposite 

 and antagonist cavity. This sort of tooth was expressively 

 named a " dens vacuus," by Linnaeus; as, for example, in his 

 character of the genus Mya, " Testae Cardines dente crasso 

 vacuo." (Fig. 88, t.) 



The Ligament is internal when it is entirely concealed from 

 view on the valves being held closed ; and it is external when 

 we see it projecting outwardly, and occupying the ligamen- 

 tal area. 



The ligament is formed of two very distinct substances. 

 One the ligament of Gray is always external, and is at- 

 tached to the upper margin of the shell : it is of the same 

 substance as the periostracum, and is somewhat analogous to 

 the ligaments which connect the bones of the vertebrata, for, 

 like them, it is quite inelastic. " In some shells it is very thick 

 and distinct ; in others, it is very thin and scarcely visible. 

 In the Pholadae it appears to be thin, and to have the shelly 

 plates imbedded in its substance." The use of this ligament 

 is to bind the valves together. 



The other substance Mr. Gray calls the cartilage, being 

 somewhat similar to the cartilage of the vertebrata. It is 

 eminently elastic, and formed of parallel series of condensed 

 transverse fibres, which are directed from the hinge of one 

 valve to the corresponding part of the other. This substance 

 is always situated within the true ligament. When the sub- 

 stances are close together, and similar in form, the cartilage 

 is attached, just below the edge of the ligament, to a protu- 

 berance on the dorsal margin of the shell, named the callus 

 or fulcrum. But when the cartilage is placed at a distance 

 from the ligament, the former is said to be internal; and it is 

 enclosed in a cavity amongst the teeth, or in one peculiar to 

 itself, as in Mya. The cartilage is easily distinguishable be- 

 cause of its black colour and pearly lustre ; and when dried 

 it is very brilliant, and has a fine display of colours. The 

 only shells that have no cartilage are the Myastropha, where 

 its place is supplied by an abductor muscle. Its use, whether 

 situated in a shelly cavity, or walled in by an inelastic liga- 

 ment on one side, and pressed upon on the other side by the 

 callus, is to open and keep open the valves. 



Imbedded in the ligament, and overlying the hinge, there 



H H 



