430 FORMATION OF SHELLS. 



whorls of Mitra episcopalis, Triton pileare, Cassis glauca, 

 Voluta hebraea, and several other shells. In some, which 

 have an elongated acute spire, as in the various species of 

 Fasciolaria, and in the turreted shells, such as the Terebrae, 

 Cerithia, and Turitellae, it entirely fills the cavity of the tips, 

 which, from their small size and original thinness, would 

 otherwise have been liable to be broken. Its deposition is 

 not confined to adult shells, for I have observed it filling the 

 cavity of the upper volutions, and lining the succeeding 

 ones, in a slit specimen of a young S trombus gigas in the 

 possession of Mrs. J. P. Atkins, to whom I take this oppor- 

 tunity of expressing my thanks for her kindness and liberal- 

 ity in allowing me to examine the numerous dissected shells 

 in her collection. 



In those Volutes which retain the nucleus (or that form 

 which the shell has when first hatched), that part which was 

 originally very thin and brittle, is speedily filled up with the 

 deposit in question. In fact, all shells whose spires are 

 exposed, and, being thin in their young state, would be 

 liable to be broken oiF by the action of the sea, have that 

 part strengthened by the internal deposition of calcareous 

 matter. 



The distinction between these and the decollated shells, 

 such as Bulimus decollatus, Cerithium decollatum, &c., is, 

 that in the latter the animal, instead of lining the upper 

 whorls with an internal coat, suddenly withdraws its body 

 from them and forms behind its extremity a concave sep- 

 tum ; and the vital communication between the body and 

 the apex of the shell being thus cut off", the latter part 

 decays, in the manner of a dead shell, and falls off in par- 

 ticles.* 



The greatest development of the deposit mentioned above 

 is to be observed in the genus Magilus, in which the young 

 shell is very thin, shaped like a Purpura and of a crystalline 

 texture ; but when the animal has attained its full size, and 

 has formed for itself a lodgment in a coral, the greater 

 part of the cavity of its shell is filled with a glassy sub- 

 stance, leaving only a, small conical space for the reception 

 of its body : layer after layer of this substance are then 

 deposited in rapid succession, in order to keep the body of 

 the animal on a level with the top of the growing coral in 

 which it is buried, until its shell is almost lost in the quan- 

 tity of glassy matter subsequently formed. 



* M. De Blainville refers the decollation of the spire to the inner surface 

 of the cavity of the shell becoming filled with a very brittle glassy deposit. 



