214 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



gradual and increasing complication of the figure 

 of the line of suture, that is to say, of the inter- 

 section of the separating partitions of the inhabited 

 parts with the external wall of the shell. These suc- 

 cessive partitions are secreted by the " mantle " of 

 the animal, the border of which, more or less cut out 

 and jagged, reproduces its form in the sutural line 

 of the shell. The more or less complicated jagged 

 edge of the mantle constitutes, it must be owned, a 

 zoological character of feeble importance, in which 

 it is difficult to see any progress for the branch or 

 any improved adaptation to its physiological needs. 

 And yet, the progressive complication of the lobes 

 and saddles of the sutural line presents itself, 

 as a rule, in all the phyla of the Ammonites, to 

 such a degree that the characteristics drawn from 

 the shape of the partitions have been chosen by 

 palaeontologists as the most essential basis of the 

 establishment of genera and of families. The com- 

 plication of the partitions seems to me a normal 

 phase, advanced or, if you like, highly specialized, 

 of the evolution of each branch of the Ammonoids. 



Almost as much might be said, but with less 

 generality, perhaps, for the uncoiling of the shell 

 of the Ammonites, a phenomenon which occurs at 

 various epochs, and in various degrees, in absolutely 

 distinct branches. I shall only quote, in the 

 Triassic epoch, the genera Ehabdoceras and Coch- 

 loceras ; in the Rhsetian epoch, the Choristocerata ; 

 in the Jurassic epoch, the Sphcerocerata, Patocerata, 

 and Baculince. But it is, above all, in the Cretacean 

 epoch, that is, towards the end of the reign of the 

 Ammonoids, that the phenomena of the uncoiling 





