THE MAMMALIA OF THE UIXTA FOUMATION". 473 



an exceedingly pi-imitive pattci-n. As far as the upper molars are concerned this is 

 no doubt tiiio, thcise teeth never vaiying from a simple tritubercular paltein. T>is- 

 sacus, however, shows that the lower molars were originally of the tuberculo-sectorial 

 pattern, as the postero-internal cusp is distinctly present, from which it follows that 

 the simplicity of the Mesonyx molars has been attained by the suppression of parts, 

 somewhat in the same fashion as has been done in the lower sectorial of ITi/<i',aodon 

 and the Felidce. These lower molars of Mesonyx can, however, hardly be called sec- 

 toiial, as the shearing action is limited by the small height of the outer tubercles of 

 the upper molars. The cusps of the lower series are, therefore, worn upon the sum- 

 mits and very rapidly become blunted and worn down. 



A second specimen, consisting of parts of two metatarsals and a phalanx, should 

 probably be referied to this species, as their shape and character are very similar to 

 those of M. ohtusidens. The latter species is remarkable for the small size and weak 

 development of the feet, but if these bones from the Uinta are correctly referred, M. 

 uintensis must have had very different proportions, for they are considerably shorter 

 and much more massive than in the lion. It is obvious that the Uinta Mesonyx must 

 have been a very formidable animal, much more so than the more highly organized 

 carnivores and creodonts (except Uein/pscdodon) which succeeded it in the White 

 Kiver epoch. 



Measurements. S/. 



Diameter of incisor (fore anil aft) Oil 



" " (transverse) Oil 



Dianu'tcr of caiilnp 033 



LoDglli sfcond lowpr premolar 014 



" fourth 028 



Tliifkncss " " " 013 



Length lower molar series f'TS 



" first molar 031 



Thickness first molar OI.T 



Length second " 02U 



Thickness second molar 015 



Length third " 018 



Transverse diameter of mandilxilar condyle 0:17 



MI ACTS Tope. 



This genus of creodonts is the one which most closely approximates the true 

 Carnivora and is characterized by the sectorial pm J and m i. This is also true of 

 Didymiclis, but here the last upper molar is wanting and the lower sectorial is of 

 very diil'erent character. 



