THE orERCULUM. 443 



the shell when the animal is drawn into its cavity. It has 

 hitherto been observed only in those Mollusea which have 

 pectinate branchiae, and in two genera (Cyclostoma and Heli- 

 cina) amongst the pneumonobranchous land shells. 



The muscle by which the animal is attached to its shell is 

 generally affixed to the hinder part of the cavity, a little 

 wdthin the mouth ; in the long spiral shells, whicli have a 

 small or moderately sized mouth, it is simple, and forms a 

 single scar on the pillar : but in those which have a large 

 mouth and a slightly developed spire, as Neritina and Nerita, 

 it is divided into two portions, one attached at each end of 

 the pillar lip ; and in those which have the mouth almost 

 as large as the cavity of the shell, such as Capulus, it is 

 divided into two nearly ec^ual parts, which extend along each 

 side of the cavity, and form what is generally called a horse- 

 shoe-shaped muscular scar. The insertion of the muscle 

 forms similar scars on the inner side of the operculum. 

 Thus in most opercula there is only a single scar : in those 

 of Nerita and Neritina there are two scars, one at each end ; 

 and in Capulus the operculum has a horse-shoe-shaped im- 

 pression. The only exception, as far as 1 am aware, to this 

 rule, is in the genus Concholepas where the muscle forms a 

 continuous band nearly round the cavity of the shell, while 

 the operculum, which is very small, is marked only with a 

 single subcentral ovate scar. 



By far the greater number of these bodies are formed of 

 a more or less condensed cartilaginous matter, similar to the 

 periostracum ; and they are often strengthened by a deposit 

 of calcareous shelly matter on their outer side, or more 

 rarely by a similar deposit of greater or less thickness on 

 their inner surface, in some few instances extending to both. 

 The deposit on the inside may be compared to the glassy 

 enamelled coat which is found in the cavities of certain 

 shells, and that on the outer surface may be considered ana- 

 logous to the hard enamelled coat covering the backs of the 

 Cowries. A few opercula, such as those of the genus Neri- 

 tina, appear to be truly shelly. 



The opercula agree with the valves of shells in being- 

 developed on the embryo while included in the egg, and in 

 increasing in size by the addition of new matter round the 

 circumference of the base of the cone of which they are 

 formed : they also agree in the cone being sometimes simple 

 and straight, and sometimes curved into a spiral form. That 

 Adanson regarded them as analogous to the valves is evident 

 from his calling the shells which are provided with tliese 

 lids on their mouths Sub-bivalves. The principal difference. 



