THE LIGAMENT AND HINGE 



27: 



strong anterior lateral becomes nearly central and simulates a 

 cardinal. 



Some bivalves, e.g. Anodonta^ Ostrea^ Pedum, many Mytilus, 

 have no binge teeth at all, in others the laterals are wanting 



Fig. 188. — Hinges of A, Cardlta semiorbiculata Brug., and B, Unio rectus Lam., 

 showing how, in inequilateral shells, the lateral teeth tend to shift their position. 

 a.m, anterior adductor, p.m, posterior adductor muscle; c, c, cardinal teeth; 

 p.l, posterior lateral teeth; /, ligament. 



(^Psammohia, Diplodonta) . In the Arcadae the hinge consists 

 of a number of very similar denticles, which are often serrated 

 like the teeth of a comb (Fig. 189). 



Hinge-teeth are probably, in origin, derived from the crenula- 



n.a 



Fig. 189. — The hinge in Arcadae : 



A, Nacula Loringi Ang. x §; 



B, Area granosa L. ; u.a, um- 

 bonal area. 



Fig. 190. — A, Tridacna scapha Lam.; B, 

 Cardium enode Sowb., showing the inter- 

 locking of the ventral margins. 



tions or ribbings of the surface of the shell, the upper ends of 

 which impinge upon the dorsal margin and mark it in a way 

 which is quite recognisable when the shell is thin. Similar 

 crenulations, resulting in interlocking of the valves, are not 



