286 POPULAR BRITISH CONCHOLOGY. 



successive layers in the same manner as tlie shell, and must 

 not be confounded with the ejpijphragm, which is secreted by 

 snails as a winter covering. Opercula differ from each other : 

 first_, in their composition, some being testaceous, or formed 

 of shelly matter ; others being corneous, or of a horny sub- 

 stance : secondly, in their growth : some are sjnral, being 

 formed of many flat whorls, others of few whorls ; some are 

 concentric, having the lines of growth in successive rings 

 round the central nucleus ; in others the nucleus is lateral, 

 and again in others terminal. 



SYMMETRICAL UNIVALVES. 



In Patella, Dentalium, and some similar univalves, the 

 cone is quite simple, not spiral, but only slightly curved 

 from front to back, not from side to side. They are there- 

 fore conical, simple and symmetrical. The Dentalium, we 

 may observe in passing, will help to illustrate the formation 

 of an ordinary spiral univalve. Tor, if we suppose the 

 slightly-carved tube soft and pliable in our hands, we 

 could begin at the smaller end, and coil it round in whorls 

 like a corkscrew, till it became a spiral shell or twisted cone. 

 Nor does it appear to be very essential with respect to the 

 nature of the mollusc, whether his shell be straight or 



