l38 Conchology. 



principal use of the shell, however, is to serve as a covering or 

 defence of the animal. 



Testaceous animals are not only extremely different in exter- 

 nal form, but also in the mode of their production. Some are vi- 

 viparous, as most of those which inhabit bivalve shells, multi- 

 valves, and even som^e of the univalves; while the others, which 

 form the far greater proportion, are oviparous. In one point, 

 however, they all agree, that whatever be the mode of produc- 

 tion, whether from an egg, or directly from the uterus of the 

 mother, the shell is formed on the body of the young animal, and 

 is proportioned to its bulk. 



The best observations which have yet been made, and the 

 most elaborate investigation which has hitherto appeared, con- 

 cerning the formation and developemeni of shells, are those of 

 the celebrated Reaumur, which were published in the memoirs of 

 the Academy of Sciences for the year 1709. The same subject 

 has been prosecuted by other authors, but their results have been 

 nearly the same as those of this distinguished naturalist. Klein 

 is almost the only author who has advanced a different opinion. 



In his dissertation concerning the formation of shells, he charg- 

 es Reaumur with supporting the opinion, that testaceous animals, 

 when they proceed from eggs, are not furnished with the shell, 

 but that it is formed after being hatched. This opinion, indeed, 

 has been ascribed to Reaumur by the historian of the academy, 

 who, in the analysis of his excellent memoir of the formation of 

 shells, has observed, ' that hitherto the curious have been struck 

 with the prodigious variety, the exact regularity of structure, the 

 singular beauty and splendor of color of shells ; but naturalists 

 have been less attentive in studying and investigating the mode of 

 their formation) They seem to have thought that although shells, 

 as well as the covering of crustaceous animals, are bones placed 

 externally to the animals which they cover, it was necessary to 

 consider them as parts of their bodies, and to include this inex- 

 phcable circumstance under that of the general formation of ani- 

 mals, which is incomprehensible to the human mind. They have 

 therefore supposed that the animal and its shell proceeded from 

 the same egg, and were developed together; and they have rested 

 satisfied in admiring the economy of nature in providing so elabor- 

 ate a covering for so low an order of animals. But this suppo- 

 sition, although probable, is not founded on truth. The animal 

 only, not the shell, is produced from the egg. The discovery of 

 this fact is owing to Reaumur.' 



It must seem very extraordinary, that such an error should have 

 crept into the abstract of the memoir of this celebrated philoso-' 



