484 TERMINOLOGY OF 



the middle as in the Ricini ; and sometimes so near the neck 

 that it plays by a kind of hinge on the margin of the inner 

 lip of the aperture. This is the case in the genus Nerita. 



In those Gasteropods which have the operculum seated at 

 a distance from the neck, or remote from the pillar lip, it 

 has its upper point towards the superior angle of the aper- 

 ture when the animal is retracted ; but when the snail pro- 

 trudes itself the position of the operculum is reversed. This 

 can only be accomplished by a complete rotation of the 

 operculum, and the process may be readily observed in 

 Purpura, in Strombus, in Buccinum, and in many other 

 operculated genera of Gasteropods. 



Opercula may be calcareous or horny. The former exist 

 in comparatively few genera, ex. Nerita, Natica, Turbo, 

 Phasianella; and these are all holosomatous and phytivorous. 



In figure the opercula vary much, the variation having 

 always a determinate relation to the shape of the aperture, 

 and the number of whorls in the spire. In some genera 

 it is very small proportionably, and cannot close the aperture. 

 This is remarkably exampled in the genus Conus. Some 

 authors designate this rudimentary operculum as spurious. 

 " We are apt, however," says the Rev. L. Guilding, " to 

 make use of this word spurious without sufficient considera- 

 tion. We should recollect, when wondering at the smallness 

 or weakness of the horny opercule of some Mollusca, that 

 the species which possess such either live under the sand, 

 reside in safety on the coasts, or quit the waters when they 

 are not feeding, the shell being held down close to the rocks 

 by a dried mucous secretion, as in some Turbinidae, or by 

 the mere adhesion of the foot, as in Purpura, &c. The 

 operculum, which in many cases would not close the ex- 

 panded aperture, is only brought into use in cases of great 

 peril, when the hold of the adhesive foot is loosened, the 

 vessels are emptied of mucus, the various secretions, or the 

 poisonous or coloured fluids by which the enemy is to be 

 driven back or baffled, and the animal retires into the nar- 

 rower whorls, for which alone the operculum is fitted. When 

 the operculum is perfectly solid and testaceous, we may be 

 sure that its possessor commonly resides in places where it is 

 subject to the sudden attacks of dangerous pursuers. Here 

 it will be of ample size, and capable of closing the larger and 

 exterior whorl. The structure and composition of this organ 

 indicate the habits of the inhabitant in so many cases, that 

 its value in generic characters is far greater than many are 

 willing to allow."* 



* Zool. Journ. v. 34. 



