No. i.] OSTEOLOGY OF PCEBROTHERIUM. 45 



was not unlike the llama, but with manifold differences. In 

 size, the Miocene genus is very inferior to the recent one. A 

 llama skeleton is about 112 cm. high at the shoulder, while 

 the tallest individuals of the large species, P. labiatum, which 

 I have seen, do not exceed 69 cm. in height. The head of 

 PcebrotJierium, and especially the face, is longer, lower, and 

 more pointed, and the neck, though much elongated, is still dis- 

 tinctly shorter than in the living genus. The thorax is longer, 

 and, owing to the lightness of the ribs, more open in appear- 

 ance ; the lumbar region, on the other hand, is rather shorter. 

 The limbs are quite as long, if not longer, and in particular more 

 slender, giving the animal quite the look of some of the ante- 

 lopes, which is increased by the fact that the metatarsals are 

 decidedly longer than the metacarpals, and the ungual phalanges 

 are long and pointed, very different from the nodules of the 

 modern Tylopoda. 



No other known genus from the White River beds had even 

 a remote resemblance to PcebrotJierium ; among European ani- 

 mals Xiphodon at once suggests itself as the one most like it in 

 appearance, though, of course, no question of near relationship 

 between these genera can be entertained. 



Structural Changes in the Cameline Series. 



We have next to consider the steps by which the Camelidce 

 have reached their present differentiation, assuming for the 

 present that the series of genera usually referred to this group 

 represent the stages in its evolution, and reserving the discus- 

 sion of relationships until the facts of structure have been pre- 

 sented. 



I. The Dentition. 



The oldest member of the Cameline series is, according to 

 Cope (No. 9), the genus Pantolestes of the Wasatch Eocene. 

 In this form the superior molars are of the primitive trituber- 

 cular type, which originally caused the reference of the genus 

 to the pseudo-lemuroids, but are complicated by the addi- 

 tion of the para- and metaconules, making five in all, but the 

 protocone is still median in position. The inferior molars cor- 

 respond in structure, being of the type which Cope has named 



