THE TERTIARY PERIOD. 243 



' The rodents were represented by several speeies. Among these 

 was a porcupine (Hystrix vetmstus), and a heaver [Castor tortus), 

 about half the size of the one now living. 



The horse family (EquiJce), were represented by at least four 

 genera and fifteen speeies. One of the most remarkable of these 

 genera was the Hipparian, which was already present in the pre- 

 vious Miocene, and was described under that head. The species 

 were comparatively small in size. Hyperion occidentalis, whose 

 remains occur at several horizons, and at widely separated localities, 

 was the largest species, but was only about the size of the ass. 

 Three other species, found on the Niobrara, and described byLeidy, 

 were still smaller. Merychippus, another genus, which occurs on 

 the Niobrara, was so named because of its large, broad grinders. 

 The name means ruminating horse, but it refers only to the resem- 

 blance, as it did not partake of the character of ruminants. Mery- 

 chippus mirabilis, the largest of the two species described byLeidy, 

 was a little larger than the ass. Protohippus, which is also repre- 

 sented in the Nebraska Pliocene by at least four species, had even 

 a more complicated structure of the enamel of the teeth than the 

 modern horse. Protohippus supremus, which in size was about 

 half way between the ass and horse, was the largest species. Pro- 

 tohippus parvulus, which was obtained by Marsh at Antelope Sta- 

 tion, Nebraska, was only about two and a half feet high when 

 mature. Closely related to the preceding was Pliohippus, of which 

 Marsh described two species. It had only one hoof to each foot, 

 but large splint bones still remained. Its principal points of differ- 

 ence from the true horse lay. in the shape of the skull, hoof, and in 

 the shorter molars. Another genus of animals from the Niobrara, 

 supposed to belong to the horse family, but which reference is un- 

 certain, owing to the paucity of the materials for determination, 

 Leidy has called Hyohippus, and under that name lias described 

 two species. The most perfect, at the least the most modern of the 

 Pliocene horses of Nebraska, was Equus cxcelsrs. Dr. Ilayden 

 first found its remains on the Loup, then on the Niobrara, and then 

 at other points. It was about the size of a medium-sized modern 

 horse, and differed only in trilling details from the present one I 

 have found its remains in the uppermost Pliocene beds in the Re- 

 publican Valley, and in the Quaternary. It extended over from 

 the Pliocene to interglacial times. This most modern of the Plio- 

 cene horses, seems to have been the culminating form of the family 



