TRANSACTIONS OF WAGNER 



44 



UINTA SELENODONTS 



the well-understood phyla an increased stature in each succeeding genus 

 appears to be the rule. 



In the second place, the relative length of the limbs and feet is aug- 

 mented even more than the enlargement of the head and body. 



In addition to these general facts, which are apparent at the first glance, 

 a detailed comparison of the two genera brings out very many more cor- 

 respondences : 



(i.) The dentition of Protylopus has nearly reached the stage of develop- 

 ment found in Poebrotheriitm, but there are many minor though significant 

 differences, all of which are in the direction of that greater simplicity 

 of structure which should characterize an ancestral form. Thus, the incisors 

 have already attained almost the same condition as in the White River type ; 

 there are no diastemata in the dental series, but what may be called incipient 

 diastemata are visible in the short interspaces between the canine and p. i, 

 and between p. i and p. 2. The premolars are of the same type as those of 

 the succeeding genus, low, elongate, acutely pointed, and trenchant, but they 

 have not yet acquired such great antero-posterior elongation, and the lower 

 ones are made simpler by the absence of the basal cusps. The molars are 

 extremely brachyodont, while those of Poebrotherium, though still short 

 crowned, show a distinct tendency towards hypsodontism. We may also 

 observe changes in the proportionate development of the molar cusps. Thus, 

 in Poebrotherium the upper molars have become more elongate antero- 

 posteriorly and narrower transversely ; the external crescents are thinner and 

 more compressed, the ribs and buttresses much less prominent, and the 

 valleys narrower and deeper. In the lower molars, the cusps are less conical, 

 more compressed and plate-like, and the valleys deeper. Protylopus departs 

 from its White River successor in approximating to the other Uinta seleno- 

 donts and to the still earlier Bridger types. 



(2.) The skull of Protylopus is so like that of Poebrotherium that the 

 resemblance strikes the observer immediately. The skull has the triangular 

 shape and long, slender, tapering muzzle which is so characteristic of all the 

 Tylopoda, but the muzzle is decidedly less elongate than in Poebrotherium ; 

 the cranium is narrower, less capacious, and the sagittal crest, though relatively 

 no longer, is higher and more prominent. The postorbital processes of the 

 frontal and jugal are shorter, leaving the orbit much more widely open behind, 

 while the auditory bulla is very small, not reaching the paroccipital process, 

 and is hollow and free from cancellous tissue. Amon? the less obvious differ- 



