450 THE OPERCULUM. 



of hinge on the sharp inner lip of the shell. The addvictor 

 muscle of this genus is divided into two portions, one placed 

 at each end of the pillar : of these the hinder is the largest, 

 and forms a submarginal scar along the end of the last whorl 

 of the operculum, while the anterior is smaller, and forms 

 an ovate scar behind the two processes. The disk to which 

 these opercula are fixed is like that of Littorina, and there 

 is a slight ridge extending the whole length of the front 

 edge of the muscle, a little anterior to it, which probably 

 secretes the shelly matter of the operculum : in this office 

 it may, perhaps, be assisted by the edge of the hinder part 

 of the mantle, situated just before it. 



I might have been inclined to regard the operculum of 

 Navicella as anomalous, had I not had an opportunity of 

 comparing it with its ally Neritina, which has enabled me 

 to explain its structure. In this genus, as in Concholepas 

 and Cryptostoma, the mouth occupies so large a share of the 

 cavity of the shell, and the hinder part of the foot of the 

 animal is so short, that the operculum cannot be folded over 

 in such a manner as to close the aperture. But instead of 

 being very small, as in the two latter genera, the operculum 

 is rather large in comparison with the size of the animal, and 

 appears to serve a new purpose, viz., to separate the viscera 

 from the upper surface of the foot, as the shelly plate does 

 in the genus Crepidula. The part which projects externally 

 is very small, and can only be compared to the flexible car- 

 tilaginous fringe on the edge of the outer whorls of the 

 operculum of Neritina ; whilst the shelly part which is in- 

 cluded in the body of the animal is four times the size of 

 the external portion, and appears to represent the anterior 

 margin and the two processes of the operculum of that genus, 

 greatly developed. The anterior process, which appears to 

 be analogous to the curved projection in Neritina, is pro- 

 duced into a straight lanceolate ridge, and the posterior into 

 a rounded strongly serrated edge ; the straightness of these 

 processes evincing that this operculum does not revolve on 

 its axis. 



Other ovate spiral opercula of few volutions have a con- 

 centrically ridged inner surface (Fig. 80, h.), and the outer 

 surface covered with a shelly coat, which varies in thickness 

 in the different genera, being thin in Nacca, Phasianella and 

 some Cyclostomata, and very thick and convex in the genera 

 Turbo and Imperator. The disk to which these opercula 

 are attached is like that of Littorina ; but anterior to the 

 muscle there is a very deep groove, into which the opercu- 

 lum can be pushed, and which probably covers the front part 



