52 THE OPERCULUM. 



Subspiral, or scarcely spiral, in Melania (Ixxi, 15), 



Multispiral, or many-whorled (ii, 36), as in Trochus^ where 



they sometimes amount to twentj'^; the number of turns 



which the operculum makes is not determined by the 



number of whorls in the shell, but by the curvature of the 



opening, and the necessity that the operculum should 



revolve fast enough to fit it constantly. (Moseley). 



Articulated, when it has a projection, as in Nerita (ii, 38). 



Radiated is a modification of the articulated operculum 



in which the spiral is not so evident. Navicella(lxxviii,'78). 



Too much importance, however, must not be attached to this 



very variable plate, as an aid to classification ; it is present in 



some species of Voluta, Oliva, Conus, Mitra, and Cancellaria, but 



absent in others ; it is (indifferently) horn}^ or shelly in the 



different species of Ampullaria and ISTatica ; in Paludina, it is 



concentric, whilst in the nearly related Paludomus it is lamellar, 



and in Cerithium it is either multispiral or paucispiral. The 



epiphragm or temporary plate with which pulmoniferous land 



snails close the aperture of the shell during hibernation may be 



distinguished from the true operculum by its homogeneity and 



want of growth-marks. 



The operculum is large and well developed in Concholepas 

 Peruvianus^ a large species of Purpuridse having a shell limpet- 

 or rather haliotus-like in its greatly expanded aperture and body- 

 whorl and comparatively small spire. The animal, like the 

 limpets adheres firmly by the suction of its foot to rocks, so that 

 an operculum is in this case an entirely useless appendage — yet 

 it is always present. 



'' The majority of the individuals of Volutharpa ampullacea 

 are without opercula, even without a trace of the pad-like gland 

 or area from which the operculum is secreted. About ten per 

 cent, of the individuals of the var. acuminata which I have 

 examined had traces of this gland or area, marked by its smooth 

 and rather whitish surface on the granulous dark slate-colored 

 foot. About fifteen per cent, had well-developed opercula in the 

 proper position. I have ascertained the same to be the case 

 with regard to the typical form, from alcoholic specimens, 

 collected by Dr. William Stimpson in Behring's Strait. There 

 is no mistake about this, strange as it may and must appear, 

 that different individuals of the same species are indifferently 

 operculate or inoperculate. 



A careful examination of this appendage reveals some singu- 

 larities in it worthy of note. At first the operculum is of an 

 ovoid form, with the nucleus near the edge at the larger end, and 

 increases by additions around the edge, but principally upon the 

 smaller or upper end. However, at some late period of its 

 growth it take^ a new start, and, seeminglj', a new operculum 



