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UINTA SELENODONTS 



much less upon the dorsal side than in the latter. The second phalanx is 

 much shorter than the first and very slender, especially transversely ; its proxi- 

 mal trochlea is less distinctly divided into two facets than in Poebrotherium, 

 but the distal trochlea is much as in that genus, describing a semicircle 

 and extending as far upon the dorsal as upon the plantar face. The ungual 

 also has its proximal trochlea less clearly divided into two facets than in the 

 White River genus, and it is rather shorter and less tapering. Another dif- 

 ference from the latter is the fact that the ungual is not quite so straight, 

 but is slightly curved, with the concavity directed towards its fellow of the 

 adjoining digit. 



The Systematic Position of Protylopus. 



In my preliminary paper I said of this genus (there called Parameryx) : 

 " There can be very little doubt that Parameryx is the direct and immediate 

 ancestor of the White River Poebrotherium, which it so much resembles, and 

 thus it holds an important place in the main line of tylopodan descent." 

 ('98, p. 75.) Wortraan had independently reached exactly the same conclu- 

 sion ('98, p. 1 10), and the foregoing description should render the point suffi- 

 ciently clear. In every detail of its structure the Uinta genus is just what 

 we should expect to find in the ancestry of the White River form. 



In many discussions of phylogenetic problems the reasoning proceeds 

 upon the assumption, expressed or implied, that the steps of evolutionary 

 change keep equal pace in the various organs, and that, consequently, if a 

 given ancestral genus differs from a descendant by a certain amount in the 

 structure of the teeth, it will also differ by an equivalent amount in the 

 structure of the feet. Sometimes this assumption is justified, but quite as 

 frequently it is not, and we find that a genus may have its teeth much more 

 modernized than its feet, or vice versa, or that the manus and pes represent 

 different stages of phylogenetic advance, which will be equalized at a later 

 time in a succeeding genus. Bearing these facts in mind, our examination 

 of Protylopus completely confirms the suggestion that Poebrotherium is its 

 direct descendant, an interpretation which requires no straining of the facts 

 to make them fit it. 



In the first place, we observe that there is marked increase in size in 

 the descendant genus, an increase which is a very common occurrence in 

 evolutionary advance. Not that a reduction in size may not occur, but in 



