UNIVALVE MOLLUSCANS. 279 



up one and opens another, as its occasions re- 

 quire. I have a very large specimen, in which 

 there are traces of eighteen apertures, and all 

 but six are stopped up. If we turn our eyes 

 from these to the Buccinidan or Whelk tribe, 

 we are struck by an open Peruvian shell, which 

 at first sight seems like a limpet, 1 but upon in- 

 quiry we find that it is defended by an opercu- 

 lum, the plan of protection being here changed, 

 and, instead of an under- valve, or a rocky muni- 

 tion, it is closed by a broad plate, which some 

 peculiarity in its structure and organization 

 doubtless required ; from this by Purpura and 

 Monoceros to the true Buccinum, the mouth 

 narrows and the operculum with it. 



If we examine the common periwinkle, we 

 find the mouth of its shell closed by a horny 

 organ called the patch, which is attached to the 

 foot or rather neck, by its convex or lower sur- 

 face, sitting on a sub-triangular flat space spi- 

 rally convoluted ; this is the operculum, and if 

 examined on either side will be found to be also 

 spirally convoluted, proving that it is formed by 

 the part on which it sits. When the animal ex- 

 pands its foot for creeping, the operculum is 

 retracted within the shell, so as to be quite out 

 of the way. If we examine the opercula of other 

 shells, we shall find that the majority of them 



Concholcpas. 



