SECRETING ORGANS OF SHELLS. 441 



In the collection of my friend Mrs. Mauger is a specimen 

 of Helix aspersa, showing a similar deformity arising from 

 the same cause ; but in this case the obstruction has been 

 produced by a young shell of the same species. The young 

 specimen is attached to the spire, to which it had doubtless 

 fixed itself during the dry season ; and not awaking from its 

 torpor so early as its older companion, the latter, when it 

 commenced increasing the size of its shell, threw its new 

 whorl partly over the smaller individual, which was thus 

 inclosed in a prison formed by its own shell. In this in- 

 stance the form of the larger specimen is not much altered ; 

 but about one half of the young shell projects above the 

 spire. 



In like manner the Cowries, and other shells which have 

 an additional coat deposited on their back by the enlarged 

 lobes of the mantle, on arriving at the adult age, cover in 

 with this coat any body which may be accidentally attached 

 to their surface. There are two specimens of Cypraea rat- 

 tus in the collection of Mr. Gaskoin, on one of which a 

 Crepidula, and on the other a Barnacle, is evidently so in- 

 closed ; and Humphreys, in the Portland catalogue, described 

 two specimens of the same shell in which he had observed 

 a similar occurrence. Such accidents appear, however, to be 

 rare, the extension of the mantle having in itself a strong 

 tendency to prevent other animals from adhering to the sur- 

 face of these shells. 



A similar occurrence may frequently be observed in the 

 shells of the genus Cymbium, the glazed coat of which often 

 includes Balani and particles of sand. The presence of such 

 bodies under the glazed coat in these shells is so constant, 

 that I am inclined to believe that the animal deposits this 

 coat with the view of ridding itself of the irritation caused 

 by the adherent sand and Barnacles rubbing against its foot, 

 as the animal of the Chinese pond muscle (Dipsas plicata) 

 deposits its pearly coat over buttons or spines which are 

 artificially introduced into its shell. 



5. On the Deposition of Shelly Matter by the Foot. 



It has been very generally supposed, that the calcareous 

 matter of which shells are formed is secreted only by the 

 mantle of the animal ; and it has consequently been taken 

 for granted, that the expanded base of the Cassides and 

 Persona?, the broad inner lip and the closed back of the 

 Cymbia, and the polished coat on the outer surface of an 



