Conchology. 145 



ber of turns with those of younger shells of the same species. 

 The two shells do not then exhibit any other difference than in 

 their thickness; and it would be the same, by comparing the 

 youngest shells, those which have just separated from the egg, 

 with the first turns of those of the same species which have been 

 reduced by breaking them to an equal diameter. The number of 

 turns or whorls of which the spire of a shell is composed, increas- 

 es very considerably the size of the shell in univalves, and one 

 turn more or less makes a great difference in their volume. Ac- 

 cording to Reaumur, the diameter of each turn of the spire is in 

 the snail nearly double that of the preceding one, and one half of 

 that which follows; but in many other shells, both marine and riv- 

 er, the last whorls of the spire, compared with the preceding ones 

 greatly exceed this proportion. In some, the external opening is 

 twelve times greater than the preceding one, and in others, it is 

 not more than eight times. This depends entirely on the increase 

 of the animal's body, and the proportion of that increase. The 

 growth of some is lengthwise, and in them the increase of diame- 

 ter is proportionably less; while others increase more in thickness 

 than in length. Those testaceous animals which have only a fev/ 

 turns in the spire of the shell, are of this description. To the 

 former, belong such as have a greater number of turns in the 

 spire. 



Those who have adopted the opinion of Klein with regard to 

 the formation of shells, have denied the separation of the animal 

 from the shell, which successively takes place near the tip in uni- 

 valves. It is indeed on this circumstance of the connection of the 

 animal with the shell, that the truth of this theory depends. Ac- 

 cording to it, the animal is attached to the internal surface of the 

 tip of the shell in univalves, and on this connection depend the 

 Increase of the shell, and even the life of tlie animal. But it is a 

 certain fact, that the posterior part of the body of the animal is 

 entirely detached from the tip of the shell; and this holds, not 

 only 'with regard to all land and sea shells which have lost the first 

 turns of the spire, and consequently those of the tip; but also in 

 a great number of other marine testaceous animals. It seems not 

 only certain, but even necessary, that this separation between the 

 animal and the shell should also take place in bivalve shells, if we 

 take a distinct and rational view of their growth. Whether this 

 separation is suddenly effected, or by a gradual process, which is 

 most probable, it seems to be sufficiently obvious, by examining 

 the internal surface of the valves. This is still mere strongly 

 confirmed by sawing univalve shells, particularly those which are 

 considerably elongated, and have a great number of turns in the 

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