lilVALVK S11KM,S. 465 



vidctl into two parts, citlicr hy a line or I)}' an alteration in 

 the character of its markings, or by a change ol' its colonr. 



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

 (Ji) of certain projections and fossnhe in the cardinal margin 

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

 })arts of the other. In a very few instances the teeth are op- 

 [)ositc>; 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 Linnjuus; as, for exanipK', in his 

 character of the genus Mya, — "Testa) Cardines dente crasso 

 vacuo." (Fig. 88, /.) 



')^\\o Ligament is internal when it is entirely concealed from 

 view on the valves l)i>ing held closed; and it is external when 

 we see it projecting outwardly, and occupying tiu- liganien- 

 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 upi)cr 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 tlu m, it is cpiite inelastic. " In some shells it is very thick 

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

 In the Pholad.'c it appears to he thin, and to have the shelly 

 plates ind)edded in its substance." The use of this ligament 

 is to bind the valves together. 



The other substance Mr. (iray 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 liganu'ut. When the sub- 

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

 is attached, just below the c(\\fc of the ligament, to a protu- 

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

 or J'lilcnim. JJnt when the cartilage is placed at a distance 

 from the ligament, the former is said to be internal; ai\<l it is 

 cnclostd in a cavity amongst the teeth, or in one ])eculiar to 

 itself, as in Mya. 'IMu; cartilage is easily distinguishable be- 

 cause of its black coloiu- and pearly lustre; and when dried 

 it is very brilliant, and has aline display of colonis. 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 walli'd in by an inelastic liga- 

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

 callus, is to open and keej) open the valves. 



Imbedded in the ligament, and overlying the hiu'^c, there 



It II 



