FOSSIL INVERTEBRA.TED ANIMALS. 459 



left most frequently only their skeleton, scales, or impression. 

 Their remains may sometimes conduct to the knowledge of the 

 genus, but rarely, if ever, to that of the species. 



The case is different with the testa of aquatic or terrestrial 

 animals, which has often been preserved untouched in the sands 

 or in the rocks, or which has not disappeared from the last 

 without leaving the trace of its internal and external forms. 



This preservation permits us to recognize the genera and 

 species, and to appreciate the degree of analogy which they 

 may possess with what is found at present in an existing state. 



But, in general, it is difficult to pass decided judgments in 

 this respect : to do so, it would be necessary to determine what 

 it is which constitutes the species, and to fix the line of demar- 

 cation between that and the variety, if any do exist. 



Observation has demonstrated, in reference to the testa of 

 animals which are provided with one, that very sensible differ- 

 ences exist — 1st, between individuals of the same species, taken 

 in the same locality ; and, 2dly, between the same species, taken 

 in different localities, either in the living or in the fossil state. 



These differences consist in the size, in the absence, presence, 

 or number of ribs, of tubercles, or of striae ; or rather, in some 

 localities, these characters are hardly visible, while, in others, 

 they are sometimes very strongly marked. 



There are sensible individual differences between the cardia 

 rustica, taken on different coasts, such as those of Rochelle, 

 of Cherbourg, of Normandy, and of Dunkirk. It is the 

 same with the fossils; and a species in the possession ofM. de 

 France (pleurotoma dentata), taken in ten different localities, 

 varies in its forms according to all the localities. Specimens 

 from the Placentine, for instance, are in general much longer 

 and less bulky than those of the environs of Paris. 



Among the shells, in the living state, many species being 

 distinguished only by their colours, and this character being 

 wanting in the fossils, these last should exhibit, and do in fact 

 exhibit, much fewer species in certain genera. 



