24 On the Form, Growth, and Construction of Shells. 



variety and richness of painting. These four genera include 

 probably the rarest and most costly shells that are known, and 

 few persons, we think, can fail to appreciate their beauty. 



Yet the shell in the Cowry and Volute is concealed within 

 the folds of the mantle ; in the Cone, it is covered by a thick 

 and rough epidermis, which has to be removed before its 

 hidden beauties are discovered. <c God's works/' writes Prof. 

 Forbes,* " are never left unfinished. None is too minute for 

 the display of infinite perfection. The microscope has exhibited 

 to our wondering eyes beauties of structure that have been 

 concealed from mortal sight for long ages. It would almost 

 seem as if only glimpses of those excellencies of creation, are 

 permitted to man to behold, whilst the full contemplation of 

 such wondrous charms is reserved for immortal and invisible 

 admirers. " 



But living mollusks not only secrete shell-matter; they 

 have likewise the power to absorb the internal convolutions 

 and columella of their shells, either completely, or until it is 

 reduced to the thinnest film. The cone removes all but a 

 paper-like portion of its inner whorls (see Plate, Vol. x., p. 245, 

 Figs. 4, 5, longitudinal and transverse section of Gonus 

 tesselatus) , and the Gyprcea (Fig. 7) often goes still further 

 in removing all trace of its axis. 



The Olivw and Neritidce likewise remove the internal spiral 

 column of their shells; and the Auriculidce, among land-snails, 

 do the same. 



This power of dissolving shell is also used by the Muricidw 

 in removing those external spines which would interfere with 

 the continued growth of the shell. 



Hermit-crabs in like manner increase the accommodation 

 of their houses by breaking away the internal axis and convo- 

 lutions of the shell they inhabit. In the writer's cabinet is a 

 Bulmius shell from the Antilles, inhabited by a land hermit- 

 crab (the Cenobita Diogenes), which has been completely cleared 

 from its columella, and made into one commodious chamber 

 within. 



Nearly all the peculiarities in the form of shells relate to 

 some special function or habit of life of the animals which 

 inhabit them. 



Perhaps one of the most important functions which 

 requirps to be provided for is that of respiration. We have 

 already seen that in many of the burrowing bivalves, the siphons 

 cause the shell permanently to gape at the end to accommodate 

 them. Again, in the univalves, the aperture of the shell is 

 usually found to be characteristic of the division to which the 

 animal belongs ; the mouth being entire in most of the 



* .British Mollvsccr, Introduction, p. xv. 



