398 DR. carpenter's researches : 



I may advantageously lengthen this explanation by em- 

 ploying more exactly the language of Dr. Carpenter. He 

 tells vis that all shells are generated in the first instance by 

 the agency of the epithelium of the mantle, the cells of 

 which have the power of consolidating themselves by draw- 

 ing calcareous matter into their interior. By successive 

 casts of this exuvial membrane, the laminae of the shell are 

 increased in numbers. " The margin only," says Dr. Car- 

 penter, "of the mantle, has the power of giving origin to 

 the outer layer of the shell, whilst its ivhole surface may 

 generate the inner. Every new production of shell consists 

 of an entire lamina of the latter substance, which lines the 

 whole interior of the old valve, and of a border or margin of 

 the former which thickens its edge. So long as the animal 

 continues to increase in dimensions, each new interior layer 

 of shell projects so far beyond the preceding that the new 

 border, composed of the outer layer, is simply joined on to 

 the margin of the former one ; so that the successive forma- 

 tions of the outer layer scarcely underlie each other. But 

 when the animal has arrived at its full growth, the new la- 

 minae cease to project beyond the old ; and as each is still 

 composed of a marginal band of the external substance, 

 attached to the edge of an entire lamina of the inner, these 

 bands must now underlie each other, being either quite free 

 as in Ostreas, or closely united to each other as in Unio and 

 most other bivalves." — The additions to the shells of the 

 Gasteropods are made upon the same plan. 



Admitting the justness of this explanation it follows "that 

 no addition can be made to the outer stratum after the 

 subjacent layer has been formed, except by a deposition 

 upon its external surface, as in Cypraea: nor can any change 

 be made in the thickness of the middle stratum after the 

 formation of the internal layer. But this last-formed inter- 

 nal layer may be thickened by successive deposits to any 

 extent ; and this appears to me to be the explanation of 

 the fact that the thickness of the internal layer at some dis- 

 tance backwards from the lip, bears a considerably greater 

 proportion to that of the middle and external layers than 

 it does nearer the margin. "Upon measuring," says Mr. 

 Bowerbank, " the relative degrees of thickness of the strata 

 at different parts of the same shell, I found the following 

 variations. The thickest part of the stratum of plates near 

 the mouth of a young Ampullaria, which were parallel to 

 the lines of growth, may be represented by the number 25. 

 At half the revolution of a whorl backwards the same stra- 

 tum was represented by 20, while the new stratum beneath 



