THE LICJAMENT AND HINGE 



273 



strong anterior lateral becomes nearly central and simulates a 

 cardinal. 



Some bivalves, e.g. Anodonta, Ostrea, Pedum, many MytiUis, 

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



Fig. 188. — Hinges of A, Cardita semiorhiculata Bnig. , and B, Unio rectus Lam., show- 

 ing how, in inequilateral shells, the lateral teeth tend to shift their position, a.m, 

 anterior adductor, ^j.?;;, posterior adductor muscle; c, c, cardinal teeth; 2'-h 

 posterior lateral teeth ; I, 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- 



FiG. 189. — The hinge in Arcadae: 



A, Xucula Loringi Ang. x f ; 



B, Area (jranosa L. ; u.a, um- 

 boual area. 



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

 Cardimn enode Sowb., showing the inter- 

 locking of the ventral margins. 



tions or ribbings of tlie 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, residting in interlocking of the valves, are not 

 VOL. Ill T 



