IX 



THE LIGAMENT AND HINGE 



27: 



strong anterior lateral becomes nearly central and simulates a 

 cardinal. 



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

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



Fig. 188. — Hinges of A, Cardita semioj^hiculata 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 (Fio^. 189). 



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



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 mteiiocking of the valves, are not 



VUL. Ill 



