30 INTRODUCTION. 



in regular layers like the rest of the shell; while the true 

 operculum is of a regularly laminated structure, having a 

 nucleus and receiving obliquely deposited additions, either in 

 a lateral spiral or concentric direction. It is attached to the 

 posterior part of the foot on the upper surface ; and when 

 the animal retires within its shell, that part of the foot enters 

 last, drawing the operculum after it, and thus closing the 

 aperture. 



The opercula of various shells differ in the first place as to 

 their chemical composition. They are calcareous when formed 

 principally of calcareous matter, like the rest of the shell, as 

 in Neritina, Nerita, and some others. They are corneo- 

 calcareous, when upon an internal lamina of horny consistency 

 there is a thickened layer of shelly matter. This is the case 

 with shells of the genus Turbo and Phasianella, which are on 

 this account distinguished from those of the genus Trochus ; 

 the opercula of the latter being horny or corneus. 



The size of the operculum is distinguished by comparison 

 with the rest of the shell ; thus, those of Strombus, Cassis, &c. 

 are small ; while those of Cyclostoma and others are large, 

 filling up the cavity at its outer edge. 



The direction in which the successive layers are deposited, 

 forms another ground of distinction. The disc is formed in 

 some instances of a series of whorls, the apex or nucleus being 

 more or less central ; if these whorls are numerous, the oper- 

 culum is described as multisplral, as in shells of the genus 

 Trochus ; if few, as in Cyclostoma, it is paucispiral. In 

 some instances the flattened spire consists of but one whorl, 

 it is then unispiral; and when scarcely one turn is completed, 

 it is described as subspiral. When the layers are applied 

 upon each other in such a manner that the nucleus is central, 

 and the edges of the subsequent layers are extended beyond 



