THE OPERCULUM. 451 



of it like a hood, when the animal is living. I have little 

 doubt that this hood secretes the thick external shelly coat, 

 which is quite out of the reach of the disk, and which in- 

 creases in size, like the rest of the operculum, by the addi- 

 tion of shelly matter to the edge of its last whorls. That 

 this is really the mode of growth of that part is proved by 

 its being in many cases marked with spiral grooves ; while 

 in others the front part of the last whorl is of a different 

 colour from the rest of the operculum, as though it had been 

 covered from the light. It has been sometimes thought that 

 such shelly opercula are attached to the animals by their 

 convex sides ; but this is not the case in any of the many 

 specimens that I have examined, in which the opercula were 

 naturally attached to the back of the animal. Some of them 

 have the outer edge of their whorls dilated and free, and 

 occasionally even elevated in such a manner as to form a 

 spiral ridge on the outer side : a structure which may be 

 seen fully developed in the very beautiful operculum of a 

 West Indian Cyclostoma (Cyclostoma mirabile, of Wood's 

 Supplement, t. 7. f. 22). 



The operculum of Nerita agrees in form with that of 

 Neritina; but differs in having no cartilage on its edge, 

 which is furnished, instead, with a groove ; in its outer sur- 

 face being covered with a thick variously formed shelly de- 

 posit, as in the genus Turbo ; and in its inner surface being 

 lined with a thick, callous, polished coat. Between the outer 

 and inner coats there exists a very distinct concentrically 

 striated horny layer, like the operculum of Littorina ; and 

 the left muscular scar is deeply grooved, like that of the 

 subannular opercula. This difference in the structure of 

 their opercula forms an excellent distinctive character be- 

 tween these two genera; as do also the differences in the 

 outer surface of those belonging to the genus Nerita between 

 various species of that genus. Thus, for instance, the oper- 

 culum of N. polita is smooth, with a transversely grooved 

 marginal band ; those of N. exuvia, N. ornata and N. chlo- 

 rostoma are granular ; and that of N. peloronta is smooth, 

 with a broad convex marginal rib. 



In the orbicular many-whorled opercula of the Trochi, 

 the outside, which is generally concave, exhibits the volu- 

 tions ; and the inside is covered with a thick polished coat, 

 marked with curved lines, produced by the successive en- 

 largements of the muscular scar, radiating from the centre 

 to the circumference. 



An examination of the animal and operculum of Trochus 

 pica (which from its large size offers a good illustrative 



G G 2 



