IX PARTS OF THE SHELL IN BIVALVES 269 



In many cases (e.g. Paludina) there is no true spiral form, but 

 the striae are concentric to a nearly central nucleus, and thus 

 give the appearance of a spiral. The evolution of the oper- 

 culum in Navieella from Nerita has already been illustrated (p. 

 4jJ). Neritopsis has a very remarkable operculum, the striated 



Pyrula Purpura Littorina Aulopoma Torinia Neritopsis Conus 



X § X 3 Strombus x 2 



Fig. 183. — Various forms of opercula. 



appendage of which locks behind the columella of the shell, like 

 the tooth in the opercula of the Neritidae. 



Terms employed to denote various parts of the Bivalve 

 Shell. — The umho^ or heak^ is the apex of the hollow cone, of 

 which each valve may be regarded as consisting. This apex is 

 usually more or less twisted : it is markedly spiral in Isocardia^ 

 Diceras, some Chama^ and especially Requienia^ while in Pecten^ 

 Lepton^ and others the spiral is altogether absent. As a rule 

 the umbones point forward^ i.e. towards the anterior end of the 

 shell. In Donax^ Nucula^ and Trigonia^ however, they point 

 backward. The umbones are generally more or less approxi- 

 mated, but in Area they are widely separated. 



An equilateral shell is one in which the umbones are more 

 or less central with regard to its anterior and posterior portion, 

 while in an inequilateral shell the umbones are much nearer one 

 end than the other. On the other hand, equivalve and inequi- 

 valve are terms used to express the relation of the two valves 

 to one another as a whole. Thus nearly all bivalve shells are 

 more or less inequilateral, but a comparatively small proportion 

 are inequi valve. 



The dorsal margin is adjacent to, the ventral margin opposite 

 to, the umbones. The anterior and posterior margins are respec- 

 tively the front and hinder edges of the shell. 



The muscles which serve to close the valves leave impressions 

 on the inner surface of each valve. These, when both muscles 

 are present, are known as the anterior and posterior adductor 



