200 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



the middle Miocene of Sansan ; after that the larger 

 forms with more complicated molars, of Villefranche 

 d'Astarac ; finally the great Mastodon longirostris 

 of the upper Miocene, and the not less mighty 

 Mastodon arvernensis of the Pliocene. 



Finally, I will set forth, with a few details, a last 

 and interesting example of this same evolution, 

 that of the Lophiodontidce, which will enable us to 

 see precisely, and to better understand, the general 

 mechanism which regulates the evolution of nearly 

 all the groups of fossil animals. The Lophio- 

 dontidae are Ungulates with an uneven number of 

 digits very abundant in Europe at the Eocene epoch ; 

 they form a small natural family composed of two 

 principal phyla, that of the Chasmotherium and 

 that of the Lophiodon, this latter comprising in 

 itself three parallel sub-branches with independent 

 evolution. The table on the opposite page indicates 

 the series of the mutations of the diverse branches 

 through the Ypresian, Lutetian, and Bartonian 

 stages. 



The Chasmotheria differ from the Lophiodons, 

 especially on account of their pre-molars being more 

 complicated, and furnished with two internal 

 mounds like the rear molars. Though contem- 

 poraneous with the Lophiodons, they have reached 

 a more advanced stage of evolution as regards 

 uniformity of structure in their dentition, that is to 

 say, the tendency to homosodonty. The branch 

 of the Chasmotheria, now well known as to the series 

 of its mutations, starts suddenly at the end of 

 the Ypresian stage with the small Chasmotherium 

 Stehlini of Cuis, and continues with gradual increase 



