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UINTA SELENODONTS 



and-aft line, in consequence of which the premaxillary has but a slight lateral 

 curvature. The anterior nares, as in Poebrotherium, have the narrow form 

 and terminal, vertical position which recurs in nearly all of the primitive 

 selenodonts, and which is so different from the long, obliquely placed opening 

 characteristic of the modern Tylopoda. 



The mandible is, on the whole, very much like that of Poebrotherium, but 

 with several minor differences, which tend away from the extreme tylopodan 

 type as exemplified by the recent members of the series. The horizontal 

 ramus is long, shallow, compressed, and very slender, with a slightly sinuous 

 ventral border ; it is less elongate than in Poebrotherium, having no edentulous 

 portion, and is not bent downward at the symphysis, as in the latter. The 

 symphysis is quite long, extending back to p T , or even to p^-. From the 

 hinder end of the symphysis the chin rises gently and regularly to the incisive 

 alveoli, whereas in Poebrotherium it is depressed and nearly horizontal. The 

 two halves of the mandible are not coossified, even in aged individuals. The 

 ascending ramus is low, even lower than in Poebrotherium, the condyle being 

 raised less above the level of the molars. The masseteric fossa differs from 

 that of the latter in being broader and less deeply impressed, in having less 

 conspicuous borders, and in descending lower upon the ramus. The coronoid 

 process differs from that of Poebrotherium much as the latter does from that of 

 the recent genera of the group. In the White River type the process is more 

 recurved and pointed, shorter and more inclined backward, than in the camels 

 and llamas. In Protylopus it is still shorter, more inclined, and recurved, and 

 is altogether like that of a true ruminant. The condyle is small and trans- 

 versely extended, not having begun to acquire the peculiar, knob-like shape 

 which is. so characteristic of the recent Tylopoda. The angle is prolonged 

 into a great hook, very much as in Poebrotherium, but as the posterior border 

 is broken away in all the specimens, I cannot determine whether it is so large 

 as in the White River genus. 



Three mental foramina are present in each ramus of the mandible ; the 

 anterior one, which is the largest and quite conspicuous, is placed beneath p^, 

 while the others, which are mere pin-holes, are situated, one beneath the 

 hinder border of p^, and the other under the interval between p T and m T . 

 In Poebrotherium the foramina are much more conspicuous and have a 

 somewhat different position ; one is beneath p T , the second beneath p-g-, 

 and the third under m T , nearly the same situation which they occupy 

 in Auchenia. 



