FORMATION OF SHF^LLS. '215 



never repaired, except as far as such repairs can be 

 made by the addition of materials from the secreting 

 surfaces of the mantle. It is found that shells may 

 be impregnated with poisonous metallic salts, such 

 as those of copper, without any detriment to the 

 animals they enclose. 



The power of secreting the materials of shell does 

 not usually extend to the whole of the surface of 

 the mantle, but is generally confined to the parts 

 near the margin, composing what is termed the 

 collar. The calcareous substance is always poured 

 out underneath the epidermis,* that is, between 

 this outermost layer of integument, and the subja- 

 cent corium, which is incorporated with the mantle, 

 and may be regarded as forming one and the same 

 organ, t 



The shape of the shell depends altogether on 

 the extent and particular form and position of the 

 secreting organ. The animal, on its exclusion from 

 the egg, has already a small portion of shell formed ; 

 and the simplest case is that in which this rudi- 

 ment of shell is a concave disk. We may conceive 

 the animal, covered by its mantle, to expand the 

 border of this organ, and extend it beyond the 



* Mr. Gray considers the external membrane of the shell, or 

 epidermis, as formed by the outer edge of the plates of animal sub- 

 stance, which have scarcely any calcareous matter in their compo- 

 sition, and which are soldered together into a membranous coat. 



t A secreting power is also, in some instances, possessed by the 

 foot, as is exemplified in some of the Gasteropoda, where it forms 

 an operculum, or calcareous covering to the mouth of the shell. 

 Mr. Gray ascertained that in the Cymbia, the Oliva, and the 

 Ancillaria, shell is deposited, and most probably secreted by the 

 upper surface of the foot, which is very large, and not by the mantle, 

 which is small, and does not extend beyond the edge of the mouth. 

 (Phil. Trans, for 1833, p. 805.) 



