492 THE MAMMALIA OF THE UINTA FORMATION. 



beginning to coalesce. These five-lolx-d upper molars are found among nearly all the 

 selenoJonts of the American and European Eocene, and the discovery of Frotoreo- 

 don confii-ms Schlosser's conjecture (No. 13, p. ^2) that the Oreodontidce have been 

 derived from animals vf\t\\ molars of this pattern, though this ancestral type is clearly 

 not what he supposed it would be, namely, the common form from which both oreo- 

 donts and Tylopoda have descended. 



Lower jaw. The incisors increase in size fi-om the median to the lateral one ; their 

 crowns are proportionately higher than in Oreodon, and of somewhat different shape, 

 in that they are more flattened and quadrate in outline and the cutting edges are 

 straighter. In the median one the cutting edge is placed nearly at right angles to 

 the lateral edges, while in Oreodon the incisors are more acute and pointed. As in 

 all the later members of this family, except Plthecistes, the lower canine has assumed 

 the form and functions of an incisor, and forms the largest of that series, while the 

 fii-st premolar has become caniniform, a transformation which is entirely peculiar to 

 this line of selenodonts. Analogous changes occur in XijjJiodontherium, where pm. 2 

 is caniniform, and in Hyplsodus whei'e the canine and first two premolars have gone 

 over to the incisor series, ])ut only the oreodonts show the enlargement of the first 

 premolar and such a change of shape, that it almost deserves the name of a tusk. In 

 Protoreodon the caniniform pi-emolar is directed upwards and outwards, scarcely at 

 all forwards ; the outer side is quite strongly convex, the inner side is divided by 

 a ridge, running down from the acutely pointed apex, into two somewhat concave 

 surfaces, of which the anterior is somewhat the larger ; the anterior and posterior 

 edges of the crown are sharp and trenchant. In Oreodon both the outer and inner 

 sides of the crown are more flattened; that is to say, in Protoreodon the transforma- 

 tion is less complete, the tooth retaining very clear marks of the fact that the func- 

 tional canine is in reality a premolar. The other premolars have considerable resem- 

 blance to those of Oreodon, but are of less complicated construction and are more 

 compressed and trenchant. Pm. 2 is a simple compressed cone, with very small 

 transverse and considerable antero-posterior diameter ; on the hinder edge is a very 

 narrow and shallow depression, Avhich is much more marked in Oreodon, but not in 

 Ayrioclmrus, where this tooth is entirely simple and reduced in size. Pm. 3 is an 

 enlarged copy of pm. 2, except that the internal cingulum is more pronounced and 

 the posterior valley somewhat larger. In Oreodon this tooth is proportionately much 

 larger, the valley is enlarged, curved inwaidly and completely enclosed ; the anterior 

 half is also much more strongly concave on the inner side. Pm. 4 is more or less 

 bi-oken in all the specimens, but enough remains to show that it was more simply 

 constructed than in Oreodon, which Leidy thus describes : " From the median point 



