VERTEBRATE LIFE IN AMERICA. 681 



Tapiravus, although most of them have been referred to Lophiodon 

 a lower Eocene type. In the Post-Tertiary, a true Tapirus was 

 abundant; and its remains have been found in various parts of North 

 America. The line of descent, although indistinct through the mid- 

 dle and upper Tertiary, was doubtless continuous in America ; and 

 several species exist at present from Mexico southward. It is worthy 

 of notice that the species north of the Isthmus of Panama appear all 

 to be generically distinct from those of South America. 



In addition to these three Perissodactyle types, which, as the fittest, 

 have alone survived, and whose lineage I have endeavored to trace, 

 there were many others in early Tertiary times. Some of these dis- 

 appeared with the close of the Eocene, while others continued, and 

 assumed strange specialized shapes in the Miocene, before their de- 

 cline and extinction. One series of the latter deserves especial men- 

 tion, as it includes one of the most interesting families of our extinct 

 animals. Among the large mammals in the lower Eocene is Limno- 

 hyus, a true Perissodactyle, but only known hei'e from fragments of 

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

 and with it is found a nearly allied form, Palmosyops. In the upper 

 Eocene, both have left the field, and the genus Diplacoclon, a very 

 near relative, holds the supremacy. The lirie seems clear through 

 these three genera, but on crossing the break into the Miocene, we 

 have apparently, as next of kin, the huge Brontotheridce. These 

 strange beasts show in their dentition and some other characters the 

 same transition steps beyond the Diplacodon, which that genus had 

 , made beyond Palceosyop>s. The Brontotheridce were nearly as large as 

 the elephant, but had much shorter limbs. The skull was elongated, 

 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, Bron- 

 totherium ; Diconodon ; Menodus (Titanotherium) ; and Me g acerops, 

 which have been found only in the lowest Miocene, east of the Rocky 

 Mountains. 



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

 rium, makes its appearance. It is one stage further on in the transition, 

 and perhaps a descendant of the Brontotheridce ; 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, 1 or even-toed Ungulates, are the most abundant 



1 Artiodadyla, a sub-order of the Ungulata, in which the third and fourth digits are 

 nearly equally developed, and their ungual phalanges are flattened on their contiguous 

 sides, so that together they constitute a symmetrical form. The axis, or middle line, of 

 the whole foot lies between the third and fourth digits. 



