2/2 



THE LIGAMENT AND HINGE 



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 solid )le 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 Ijoth fully de- 

 veloped, they work in opposition io 

 one another, the ligament, l)y its 

 resistance to compression, prevent- 

 ing any straining of the adductor 

 muscles when the valves are open, 



^'''- l^iT^'"°f "W/f'^ "f ?•' and the cartilage, for the same 



and B, left valve of Mnlinm eduliti _ ® ' 



King; ca, cardinals ; l.a, anterior rcaSOU, preventing the Ventral mar- 

 laterals ; l.p, posterior laterals ; ■ f 4-i ^ „i ^n i- ^i,„- 4- 

 /, fossette ;., cartilage ; ;, liga- M"^^ of the shcll from cloSlUg too 



ment. rapidly upon one another when the 



valves are Ijeing shut. 

 The Hinge. — The valves of Pdccyi^oda are generally articu- 

 lated, below the umbones, l)y a liinge, which is furnished, in 

 the majority of cases, with interlocking teeth, small pits or 

 depressions in each valve corresponding to the teeth in the 

 other. The teeth are distinguished as cardinal, or those imme- 

 diately below the lunbo, and lateral, or those to either side of 

 the cardinals, the latter being also distinguished as anterior and 

 postei'ior laterals, according as they are l)efore or behind the 

 umbo (Fig. 184). In shells wdiich 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 

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

 Cardita, Unio, Fig. 188). In many Cham a the cardinals are 

 jnished up into the luubo and become a mere ridge, while the 



