PARTS OF THE SHELL IN BIVALVES 



269 



111 many cases (e.g. Taludina') there is no true syjiral form, hut 

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

 give the appearance of a s])iral. The evolution of tlie oper- 

 culum in Navieella from Ncrita has already l)een illustrated (p. 

 10). Neritoiisis has a very remarkahle operculum, the striated 



I'yrulu l'uriJUi;i LittoriuH Aulniionni Toriiiia 



Flu. 183. — Various forms of ojjercula. 



Noritopsis Conus 



Stroiiibus X- 



appendage of wliicli locks l)eliind the columella- of the sliell, like 

 the tooth in the (ipei'cula of the Neritidae. 



Terms employed to denote various parts of the Bivalve Shell. 

 — The umljo, or hcak, is the apex of the hollow cone, of wliich 

 each valve may l)e regarded as consisting. This apex is usually 

 more or less twisted : it is markedly spiral in Isocardia, Diceras, 

 some Cluuna, and especially Requienia, while in Fectcn, Lepton, 

 and others the spiral is altogether aljsent. As a rule the 

 umhones point forward, i.e. towards the anterior end of the 

 sIk'H. In Donax, Nucida, and TrUjonia, however, they point 

 backward. The umljones are generally more or less approxi- 

 mated, l)ut in Area tliey are widely separated. 



An cqailateral shell is one in wliich the uml)ones are more 

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

 while in an inequilateral shell the uml)ones are mucli nearer one 

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

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

 to one another as a whole. Tlius nearly all hivalve shells are 

 more or less inequilateral, l)ut a comparatively small proportion 

 are inequivalve. 



The dorsal margin is adjacent to, the ventral marfjin opposite 

 t(j, the umhones. The anterior ix\\<\. 'jiostcrior margins are respec- 

 tively the front and hinder edges of the shell. 



The muscles which serve to close the valves leave im^ircssions 

 on tlie inner surface of each \;d\e. These, when both muscles 

 are present, are known as the anterior and iiostcrlor adductor 



