FORMATION OF SHELLS. 899 



it measured 13 ; total 33. At a whole revolution backward 

 tlie upper stratum measured 18, and tlie lower or new one 

 20 ; the older of the two thus appearing to decrease gradu- 

 ally in thickness as the younger grew in substance. Upon 

 examining, in a like manner, an adult specimen of Bulimus 

 oblongus, the proportions of the strata near the lip were as 

 follows: — Outer 10, central 14, inner 17 ; total 41. But at 

 one revolution backwards they were — outer 5, central 5, 

 inner 19 ; total 29." This fact harmonizes well with the 

 preceding explanation, for it simply indicates tliat whilst 

 the thickness of the whole shell undergoes a considerable 

 increase near the lip, that increase is due to the greater de- 

 velopement of the outer and middle layers, the inner layer 

 being positively thinner than it is at a distance from the 

 lip, where it has been thickened by successive deposits. I 

 can see no reason to attribute the foregoing differences in 

 the relative thickness of the three layers at different parts 

 of the shell to any absorption or removal of the outer layers, 

 as Mr. Bowerbank seems inclined to do, for it seems im- 

 possible to imagine that the external and middle layers can 

 be thinned by absorption without the removal of the inner 

 layer, since any absorbent action must take place from 

 within, being effected by the surface of the mantle. I am 

 far from denying that such absorption does take place ; but 

 the explanation is not applicable to the facts just cited, 

 which seem to me to indicate clearly that the formation of 

 the inner layer is progressive, and not completed at one 

 effort. The truth appears to me to be, that whenever an 

 addition is made to the shell, the outer and middle layers 

 are simply joined on to the edges of the old margin, but 

 that the internal layer is carried backwards for a certain 

 distance into the interior of the shell, where the new forma- 

 tion forms a lining to the old, and increases its thickness 

 just as in the bivalves. I have never been able, however, 

 to trace it very far back, and it certainly can seldom or 

 never line the whole shell, as it does in most bivalves. But 

 this new layer seems to cover that part of the internal sur- 

 face which is in contact with the moving parts of the animal ; 

 and thus serves to prevent that irregularity which could 

 scarcely fail to exist were the new internal layer, like tlie 

 middle and external, simply joined on to the edge of the 

 preceding."* 



* Dr. Carpenter's Essays on tlie Microscopic Structure of Shells are 

 contained in the Reports of the British Association for the Advancement ol' 

 Science, 1843, p. 71 ; 1844, p. 1—23 ; 1847, p. 93—117 ; and in Ann. and 

 Masf. N. Hist. xii. 377—80. 



