35 



endeavored to trace, there were many others in early Tertiary 

 times. Some of these disappeared with the close of the 

 Eocene, while others continued, and assumed strange special- 

 ized shapes in the Miocene, before their decline and extinction. 

 One series of the latter deserves especial mention, as it includes 

 one of the most interesting families of our extinct animals. 

 Among the large mammals in the lower Eocene is Limnohyus, 

 a true Perissodactyle, but only known here from fragments of 

 the skeleton. In the next higher beds, this genus is well 

 represented, and with it is found a nearly allied form, Pakeo- 

 syops. In the upper Eocene, both have left the field, and the 

 geuus Diplacodon, a very near relative, holds the supremacy. 

 The line seems clear through these three genera, but on 

 crossing the break into the Miocene, we have, apparently as 

 next of kin, the huge Brontotheridai. These strange beasts show 

 in their dentition and some other characters the same transition 

 steps beyond Diplacodon, which that genus had made beyond 

 Pakeosyops. The Brontotheridce were nearly as large as the 

 Elephant, but had much shorter limbs. The skull was elon- 

 gated, and had a transverse pair of large horn-cores on the 

 maxillaries, in front of the orbits, like the middle pair in 

 Dinoceras. There were four toes in front, and three behind, 

 and the feet were similar to those of the Rhinoceros. There 

 are four genera in this group, Brontotherium ; Dieonodon ; Meno- 

 dus (Titanotherium) ; and Afegacerops, which have been found 

 only in the lowest Miocene, east of the Rocky Mountains. 



In the higher Miocene beds of Oregon, an allied genus, 

 Chalicoiherium, makes its appearance. It is one stage further 

 on in the transition, and perhaps a descendant of the Bronto- 

 theridce; but here, so far as now known, the line disappears. 

 It is a suggestive fact, that this genus has now been found in 

 Western America, China, India, Greece, Germany and France, 

 indicating thus, as I believe, the path by which many of our 

 ancient mammals helped to people the so-called Old World. 



The Artiodactyles, or even-toed Ungulates, are the most 

 abundant of the larger mammals now living; nnd the group 



