462 H. F. O shorn — Mammalia in North America. 



pseudo-rhinoceroses, the Hyracodons and Amynodons ; all 

 these forms present the true Rhinoceros molar pattern, but 

 they diverge most widely in the structure of the anterior teeth 

 and of the feet. The Hyracodons first appear in the numer- 

 ous and diversified Hyrachyus of the Bridger, some of which 

 exhibited rudimentary horns upon the back part of the nasals 

 (Colonoceras) ; they retained a full set of equal-sized incisors 

 and canines, and acquired a horse type of skull, skeleton, and 

 locomotion. Scott has well named them the " cursorial rhi- 

 noceroses." Colonoceras probably did not, as Marsh has sug- 

 gested, branch off into Diceratherium, for the horns of this 

 true rhinoceros are developed at the ends of the nasals ; the 

 Hyrachyinee sent off as a side branch the deer-like Triplopus 

 of the Washakie, and terminated in the Hyracodons of the 

 lower Miocene. 



The Amynodons, at the time of their discovery by Marsh, 

 were naturally supposed to be the long-sought Eocene rhinoce- 

 roses, but I have shown that no Amynodon can fill this role. 

 Garman's discovery of the skull of the remarkable Miocene 

 Metamynodon tended to confirm my views, and I have now to 

 report the discovery of many skulls and a nearly complete 

 skeleton by the American Museum Expedition. This proves 

 that the Amynodontidse were remarkable side forms. In wide 

 contrast with the true rhinoceroses, the upper and lower 

 canines develop into huge, partly recurved tusks, like those of 

 the boar. As in Elasmotherium, the premolars become greatly 

 reduced, and the molars tend to hypsodontism. The lower 

 molars are long and narrow, like those of the anomalous Cadur- 

 cotherium of the Oligocene of Europe — it is thus rendered 

 probable that Cadurcotherium is not a sloth, as Filhol has sug- 

 gested, but is an aberrant rhinoceros, related to, if not identi- 

 cal with, the Amynodons. The hypsodontism in some Meta- 

 mynodon teeth is accompanied by a partial loss of enamel. To 

 complete the aberrant character of this family, we find that it 

 has four equal-sized and completely functional toes in the fore- 

 foot, like those of the Titanotheres, not with the fifth toe re- 

 duced as in the contemporary Aceratheria. 



The true Rhinoceroses, we remember, are distinguished by 

 the entire loss of upper canines. Wortman has just reported 

 finding rudimentary upper canines in both the milk and perma- 

 nent dentitions of the older Miocene species. The true rhi- 

 noceroses suddenly appear in the lower Miocene of America 

 and Oligocene of Europe ; we have not yet traced them back. 

 In a collection of lower Miocene skulls recently obtained for 

 the American Museum we find that the premolars are still very 

 simple. In the higher Oreodon beds all traces of the superior 

 canine are lost, and the premolars have become more like the 



