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47 



UINTA SELENODONTS 



ing changes in a phylum, each step coming in every detail just so much 

 nearer to the final result. A study of the horses led me to conclude that 

 while, as a rule, development is remarkably unswerving in a large sense, yet 

 in minor details it may pursue a more or less zigzag course. ('94, p. 120.) 

 This conclusion is strengthened by what we believe to be the facts of ty- 

 lopodan evolution. No difference between the earlier genera of this series 

 and its modern representatives is more obvious and striking than in the 

 character of the canine teeth. In Protylopus and Poebrotherium the canines 

 are very small and may almost be called incisiform, but from Gomphothcrium 

 onward these teeth become larger and larger until the formidable lacerating 

 apparatus of the modern type is reached. Even though we should exclude 

 Homacodon and Tvigonolestes from the series, the analogy of all the ungulate 

 groups, condylarth, amblypod, perissodactyl, and artiodactyl, would justify us in 

 assuming that the ancestors of Protylopus possessed canines which were of fairly 

 large size and effective as weapons. If this be true, then the canines first dwin- 

 dled to very small proportions, only again to enlarge and become formidable. 

 Another instance of much the same kind is afforded by the history of 

 the premolars. In Protylopus these teeth are very moderately elongated in 

 the antero-posterior direction, and in general form resemble those of the con- 

 temporary selenodont genera and of the White River Leptomeryx. In Poe- 

 brotherium the premolars have become greatly elongated, accompanying the 

 elongation of the muzzle, and, as it were, preventing the formation of diaste- 

 mata. In their antero-posterior length the premolars of Poebrotherium recall 

 those of Xiphodon. In the John Day, however, this tendency is changed, 

 and the premolars of Gomphotherium, in their form, revert almost to the 

 Uinta type, while Procamclus and the subsequent genera of the phylum 

 are remarkable for the reduction of their premolars both in size and number. 

 These facts are very significant and have a wider bearing than merely upon 

 the phylogeny of the Tylopoda. 



(?) Leptotragulus Scott and Osborn. 



? Parameryx Marsh, Amer. Journ. Sci., 3d Sen, xiv., p. 364 (nomen nudum). 

 Leptotragulus S. and O., Proc. Amer. Phil. Soc, 1887, p. 258. 

 ? Parameryx Marsh, Amer. Journ. Sci., 3d Ser., xlviii., p. 269. 

 Parameryx Wortman, Bull. Amer. Mus. Nat. Hist., x., p. 103. 



In view of Wortman's recent attempt to rehabilitate the name " Para- 

 meryx" it will be necessary to say a few words concerning the terms which 



