166 Marsh — Principal Characters of the Protoceratidce. 



The resemblance in the two skulls is further enhanced 

 by the absence of upper incisors and the presence of large 

 canine tusks, forming together a striking similarity in impor- 

 tant features, between skulls pertaining to animals of two dis- 

 tinct orders, and from widely different geological horizons. 

 The skull of the male Protoceras is shown in Plate II, and 

 that of Dinoceras in the text below. 



Figure 1. — Skull of Dinoceras mirabile, Marsh ; type; seen from the side. 

 One-seventh natural size. Eocene. 



It is a noteworthy fact, that in still another order of ungu- 

 late mammals, the Perissodaciyla, horn-cores in pairs early 

 made their appearance, although none are known in the recent 

 forms. One of the earliest instances is seen in the genus Colo- 

 noceras from the middle Eocene, which had rudimentary pro- 

 tuberances upon its nasal bones, as represented below, in figure 

 2. The gigantic Brontotheridm of the lower Miocene all had 

 prominent horn-cores on the maxillary bones, somewhat like 

 those of the male Protoceras. One of the most unexpected 

 examples, however, in this order, appears in the Miocene 

 genus Diceratherium, the type specimen of which is shown in 

 figure 3. This animal, although a true rhinoceros, had a pair 

 of horn-cores on the nasal bones, while all other rhinoceroses, 

 living and extinct, are either without horns or have them on 

 the median line. In short, horns in pairs are unknown in exist- 

 ing mammals, except in the artiodactyles, an order of later devel- 

 opment, but now the dominant group of ungulate mammals. 



