1892.] NATURAL SCIENCES OF PHILADELPHIA. 433 



tetartocone and the corresponding tooth in the lower jaw is com- 

 pletely so. 



The line ends with the extraordinary genus, Titanotheriuvi, of the 

 lower White River horizon, in which V2i^^Aa have added the second 

 internal cusp and thus become molariform, though the difference in 

 the form of the external cusps becomes even more marked than in 

 the older genera of the series, for, although those of the premolars 

 have become somewhat flattened, those of the molars have become 

 excessively concave and are separated by a very prominent fold of 

 enamel which projects much more strongly than in Palceosyops. 

 Another difference between the two classes of teeth is the much 

 smaller size of the premolars. In the lower jaw, also, pVand * have 

 become molariform, in a way that is unnecessary to describe, since 

 it is the same as that already given in the case of p* for Lamhdothe- 

 rium and Telmatherium. 



IX. ARTIODACTYLA. 



The members of this group never attain the high degree of homo- 

 dontism which is characteristic of all the later perissodactyls, and 

 yet they display a considerable amount of premolar complication 

 and one which is brought about in very different ways in the differ- 

 ent groups of the order, there not being the same uniformity in the 

 steps of differentiation which we have found among the perisso- 

 dactyls. 



The most primitive type of artiodactyl dentition is that of Pan- 

 tolestes, of the Wasatch, a genus, the true systematic position of 

 which was made clear only by the discovery of its foot-structure. 

 Previously it had been referred to the mesodonts and creodonts. In 

 Pantoledes brachydomus, the only species in which the feet are 

 known, Pi and ^ have not been found, but Pa and ^ agree in structure, 

 having one external and one internal cusp, the proto- and deutero- 

 cones respectively. The lower premolars are extremely simple. 

 p2 — * are elongated in the antero-posterior direction and have 

 compressed, trenchant crowns, which are made up of the large pro- 

 toconid, with minute but distinct and sharp para- and metaconids. 



From the Pantoledes type of dentition many divergent lines may 

 be traced. We may begin our examination with the Oreodontidoe, 

 an extremely peculiar American family, of which the most ancient 

 member known is the Uinta genus, Protoreodon. Here the upper 

 premolars are very simple, having a compressed protocone with 



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