NERITINA. 45 



the Nerites is horny, covered on both sides with a 

 hard shelly coat. The position of the horny oper- 

 culum is shown by a groove in the edge between 

 the two coats ; and if a knife is inserted, the coats 

 can be separated from the operculum. 



As the periostraca is essential to the structure of 

 the shell, and is always present, some shells being 

 formed of scarcely any thing else, so it is with the 

 operculum, the horny part similar to the periostraca 

 of shells being always present, and forming its es- 

 sential part, and a shelly coat being in some in- 

 stances added to the outer surface, as in Turbo and 

 Phasianella, or to the inner surface, as in this genus, 

 in which the horny part is very thin and scarcely 

 visible, except where the shelly coat is very thin, as 

 at the flexible edge. 



These animals absorb the septa which separate the 

 whorls of the spire, when they have arrived at their 

 full size, so as to allow more room for the spiral 

 body, without increasing the size of the shell ; and 

 this can be done without endangering the strength 

 of the shell, as only a very small part of the whorl 

 is exposed on the surface. A similar absorption is 

 to be observed in many AuricuUdcB, and to a less 

 extent in the Cones, where the septa are only reduced 

 in thickness. (See Phil. Trans. 1833, p. 798.) 



This absorption is only superficial, and produced 

 by that portion of the surface of the mantle which 

 lies close to it, and is not to be confounded with the 

 absorption of the bones of vertebrated animals, where 

 it is produced by vessels which ramify in the sub- 

 stance of the bone, and which are accompanied by 



