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 middle 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 disappeared with the close of the Eocene, while others continued, and assumed strange specialized 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, Palæosyops. In the upper Eocene, both have left the field, and the genus 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 Brontotheridæ. These strange beasts show in their dentition and some other characters the same transition steps beyond the Diplacodon, which that genus had, made beyond Palæosyops. The Brontotheridæ 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, Brontotherium; Diconodon; Menodus (Titanotherium); and Megacerops, which have been found only in the lowest Miocene, east of the Rocky Mountains.
In the higher Miocene beds of Oregon, an allied genus, Chalicotherium, makes its appearance. It is one stage further on in the transition, and perhaps a descendant of the Brontotheridæ; 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
- 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.