Forms^ Composition^ and Structure of Shells. 34«1 



increases in thickness and circumference. This, we feel per- 

 suaded, is not the case ; and is disproved by the examination 

 of every univalve, and not supported by the structure of any 

 bivalve. The first-formed whorls of the former are not thicker, 

 not even in fact so thick, as the ultimate ones are ; nor do any 

 bivalves, so far as we are aware, gradually thicken from the 

 beak to the margin. The layers are only partial formations, 

 and do not extend beyond the points to which the animal can 

 protrude or retract the collar or the margins of the cloak. 



Bivalve shells are formed in precisely the same way as uni- 

 valves ; but here the margins of the cloak or mantle which 

 embraces the body of the animal on each side perform the 

 functions of the collar. 



You will have observed that the inner surface of all shells 

 is very smooth, and apparently denser and harder than the 

 outer. It appears that the Mollusca, to form this inner layer, 

 excrete the lime in a nearly pure state, that is to say, mixed 

 with much less animal matter, so that in concreting the par- 

 ticles become very close and compact, and receive a polish 

 from the repeated frictions of the soft parts. This structure 

 is very brittle, and hence is called vitreous. It is often used 

 to strengthen and finish the apertures of univalves, as is very 

 remarkable in the helmet shells ( Cassis Lamaixk). The upper 

 part of the spire of some turreted shells are filled with the 

 same matter at a late period of the animal's life, and, be- 

 coming dry and brittle, it is readily broken off, giving rise to 

 what are called decollated shells. The particles which divide 

 multilocular shells into several chambers are likewise vitreous, 

 and seem to be formed by a deposition from the posterior part 

 of the inhabitant at stated periods in its growth, the periods 

 being regulated by a law of its original constitution. 



In a great number of Mollusca it would appear that the 

 increase of the shell from birth to mature size is uninterrupt- 

 edly progressive ; but there are, perhaps, an equal number, in 

 which the animal, at certain and determined intervals, forms 

 a transverse rib or varix, and seems to become for a season 

 inoperative. These ribs vary much in their numbers, in their 

 figure, and in the distances at which they are placed in dif- 

 ferent shells, but in the same species are alike and uniform, so 

 that it would perhaps be better at once to refer their form- 

 ation to a law of their individual life imprinted on them by 

 their Creator, than to seek for its explanation in causes 

 which are only partially applicable, or of doubtful existence. 

 We may suppose, indeed, with Blainville, that, during the 

 season of love, the derivation of fluid and of energy to the 

 generative system may diminish the secretion of fluid and of 



