140 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



to two of these faunas, the one from the serpen- 

 tine green sandstone of the Turin hills, or Helvetian 

 stage, and the other from the blue clay of the Tor- 

 tonais or Tortonian stage ; the first comprises 120 and 

 the second 122 species or varieties. But it is easy 

 to see by examining the plates, and thanks to the 

 care which the author has taken to draw his species 

 side by side in sections or groups of natural af- 

 finities, that there is here no question of real species, 

 independent from the point of view of genetic 

 relations, but of simple forms which pass by ordered 

 transitions from one to the other when they belong 

 to the same group, but are separated by a serious 

 gap from the forms of the neighbouring group. A 

 naturalist less scrupulous than Bellardi in the art 

 of distinction might easily, and no doubt with 

 advantage, reduce the number of the species of 

 Nassa to some twenty for the Helvetian, and some 

 thirty at most for the Tortonian, with numerous 

 varieties. No doubt we are indebted to Bellardi 

 for having made known to us, by excellent drawings, 

 all these numerous varieties of Nassa found in 

 Piedmont ; we are thus able to arrive at a more 

 precise idea of the limits of variation of each species. 

 But the work would have been more interesting and 

 more philosophical if the author had reduced the 

 species to the number strictly necessary, or, to 

 express it better, to the real number, by grouping 

 round each great species a certain number of varieties 

 that he might have designated by the third name 

 of a trinominal nomenclature. 



As to the Jurassic period, an eminent geologist 

 of Lyons, F. Fontannes, has described with minute 



