

PHYLETIC BRANCHES AMONG THE VERTEBRATES 183 



dentition of the Miocene Mastodon. Having regard 

 to all these different points, the Palceomastodon is a 

 type of primitive characteristics at all points, such, 

 in a word, as we should have imagined the hypo- 

 thetical ancestor of the branch of Mastodons to 

 have been. Perhaps, even, the branch may be 

 followed down as far as the Eocene in which there 

 has been discovered another Ungulate, the Mceri- 

 therium, whose cranial characteristics are rather in 

 conformity with a generalized proboscidian type, but 

 with six incisors in the upper and four in the lower 

 jaw, the second pair of which tends to develop in 

 the form of small tusks with a sub-vertical direction, 

 instead of being projected forward as in the Palceo- 

 mastodon. But, for want of transitional types, there 

 still exists some uncertainty as to the precise genetic 

 relations of these two genera. 



In any case, we are now able to follow, save for 

 a slight discontinuity in the higher Oligocene, one 

 of the branches of the Proboscidians from the 

 Mastodon arvernensis of the Pliocene down to the 

 Palceomastodon Beadnelli of the Middle Oligocene. 

 In all this long geological journey, which comprises 

 much more than half of Tertiary times, the charac- 

 teristics of the branch become only slowly modified 

 even as regards the details of the structure of the 

 molars, the number and direction of the tusks, etc. 

 We are very far from those rapid and radical 

 transformations, from those manifold transitions 

 from genus to genus and from family to family, 

 which have been so greatly overdone in phylo- 

 genetic works on the Tertiary Mammals, 



