THE OPERCULUM. 453 



eventually covered by a green coat, which is deposited by the 

 surface of the muscle, and the part of this latter coat which 

 is left exposed as the operculum turns round on its axis, is 

 again covered by a second thin black shining coat, deposited 

 by the posterior part of the disk, or that which is formed of 

 the elevated portion of the back of the foot. 



That the spiral opercula actually revolve on their axes is 

 proved by the manner in which these coats are deposited, as 

 well as by the circumstance that the front edge of the last 

 volution is always directed towards the hinder part of the 

 inner lip of the mouth of the shell, — a position which it 

 could not constantly retain, in conformity with the manner 

 in which these opercula enlarge, without undergoing this 

 revolving motion. A convincing proof that the green coat 

 of the muscular scar which occupies the outer edge of the 

 front part of the penultimate, and the hinder part of the 

 last whorl has covered all parts of the operculum, except the 

 front half of the last whorl, which in turning has not yet 

 been brought under its influence, may be readily obtained 

 by scratching off the thin black coat, when the green will be 

 found beneath it in all parts except that last referred to. 

 The surface of the scar itself also demonstrates the fact of 

 the revolution, its hinder edge being raised and definite ; 

 while its front edge, which is progressive, is double, the 

 posterior of the two portions being elevated, and the ante- 

 rior, to which the muscle is just becoming attached, being 

 thin and ill defined. 



It has been often supposed that shells which have a toothed 

 mouth never have any distinct operculum ; but the excep- 

 tions as regards annular or spiral opercula are numerous ; 

 thus Helicina aureola and H. depressa, and all the Poly- 

 odonta, as P. clangulus, &c, have large and regular opercula. 



The genus Vermetus has an orbicular horny concave oper- 

 culum like that of a Trochus, but differing in having a large, 

 orbicular, irregularly grooved, muscular scar, placed in the 

 centre instead of on the anterior side. The outer surface in 

 some of the smaller species, as in V. dentifer,* and V. Aclan- 

 soni, is furnished with very close spiral elevated laminae ; but 

 in the large species, as V. maximus,-j' it is homogeneous and 

 horny, and does not exhibit any of the volutions. I have no 

 means of ascertaining whether these opercula revolve, but I 

 am inclined to believe that the spiral ones most probably do. 



To these, which every zoologist has been in the habit of 



* Sowerby, Genera of Shells. Serpula, fig. 0. 



t Gray, Spie. Zool. tab. 5, fig. 3, a. 



