2/2 



THE LIGAMENT AND HINGE 



CHAP. 



toothless, the ligament is folded into a number of transverse 

 ridges, which fit into corresponding grooves in the shell. 



The ligament proper is inelastic and insoluble in caustic 

 potash. The cartilage is very elastic, composed of parallel 

 fibres, slightly iridescent, and soluble in caustic potash. 



The operation of the ligament — using the word as including 

 the whole ligamental process — is in opposition to that of the 



adductor muscles. When the latter 

 close the valves, they compress the 

 ligament, an action which its elas- 

 ticity resists: thus its operation 

 tends in part towards keeping the 

 valves open. But when ligament 

 and cartilage are both fully de- 

 veloped, they work in opposition to 

 one another, the ligament, by its 

 resistance to compression, prevent- 

 ing any straining of the adductor 

 muscles when the valves are open, 

 and the cartilage, for the same 

 reason, preventing the ventral mar- 

 gins of the shell from closing too 

 rapidly upon one another when the 

 valves are being shut. 

 The Hinge. — The valves of Pelecypoda are generally articu- 

 lated, below the umbones, by a hinge which is furnished, in the 

 majority of cases, with interlocking teeth, small pits or depres- 

 sions in each valve corresponding to the teeth in the other. 

 The teeth are distinguished as cardinal^ or those immediately 

 below the umbo, and lateral^ or those to either side of the car- 

 dinals, the latter being also distinguished as anterior and pos- 

 terior laterals., according as they are before or behind the umbo 

 (Fig. 184). In shells which are tolerably equilateral there is no 

 difficulty in distinguishing between cardinal and lateral teeth. 

 But when they are very inequilateral, the whole hinge may 

 share in the inequality of growth, and an anterior lateral may 

 be thrown backward and simulate a cardinal, or a cardinal 

 may be thrown backward and simulate a posterior lateral (e.g. 

 Oardita., Unio^ Fig. 188). In many Chama the cardinals are 

 pushed up into the umbo and become a mere ridge, while the 



Fig. 187. — Hinge of A, right valve, 

 and B, left valve of Mulinia 

 edulis King; ca, cardinals; La, 

 anterior laterals; l.p, posterior 

 laterals ; /, fossette ; c, cartilage ; 

 I, ligament. 



