SPECIES AND THEIR ORIGIN 311 



remains, which have been named Meritherium, are 

 found in Eocene l formations. 



In the same strata are found also fossils of another 

 type which has been called Paleomastodon. In this 

 form, we find the same sort of molar teeth, but the 

 tusks of the upper jaw are much longer than those 

 of the lower. In the Miocene era, much later than 

 Eocene, we find the Tetrabelodon, which is charac- 

 terized by a very great horizontal extension of the 

 long upper incisors, now to be called tusks, and a 

 corresponding extension, not of the lower incisors, 

 but of the lower jaw itself. The head of this animal 

 was apparently extended in a long bony projection, 

 composed of the two tusks and the lower jaw, on 

 which doubtless rested an extension of the upper lip 

 in the form of a snout or trunk. Back in the jaws, we 

 find the typical molars. In America, we find quanti- 

 ties of the skeletal remains of Mastodon, a huge 

 elephant-like creature in which the lower jaw has 

 greatly shortened, the lower incisors have disappeared, 

 and the upper ones (the tusks) are enormously en- 

 larged, curving up and inward. Back in the jaw 

 we find the same type of molars, reduced in number 

 and increased in size. Finally, in the modern 

 elephant, we have an animal with a greatly " fore- 

 shortened " skull, in the lower jaw of which there is 

 only room for one single ribbed molar tooth at a time. 

 The upper incisors or tusks are large, and curve 

 downward, and between them is the long sensitive 



1 The Eocene period, the earliest of the Cenozoic era, dates back 

 probably three million years. 



