150 Annals of the Carnegie Museum. 



generic definition by important additions {I.e., p. 398). Still later 

 Douglass and Peterson again published papers, the one by Douglass 

 bearing especially on the history of this genus and also describing 

 three additional species P. hatchcri, P. grandis, and P. hollandi. 

 Previous to these later publications a number of important papers 

 had appeared^ describing in considerable minuteness the cranial 

 features of the different species, but no detailed osteological descrip- 

 tion of the skeleton as a whole has hitherto been published, except 

 that by Professor W. B. Scott, ^ which was to some extent based upon 

 disassociated material. Aside from the characters of the skull the 

 means for instituting a comparison of the osteological features of the 

 various species are limited. As the skull is, however, a comparatively 

 trustworthy index, it seems that with our present knowledge it is 

 possible to divide the genus into two series, the one composed of 

 dolichocephalic, the other of brachycephalic forms. The dolichoceph- 

 alic series includes P. snperbus, P. leidyi, P. macrostegiis, P. mon- 

 tanus, P. minor, P. hatcheri, and P. grandis; while the brachycephalic 

 series comprises P. cheJydra, P. carrikeri, P. vantasselensis, P. tem- 

 poralis, and P. hollandi (the two latter being rather subbrachycephalic). 



The completeness of the material in the Carnegie Museum, repre- 

 senting not only P. carrikeri, but various other species, affords oppor- 

 tunity not only for accurate comparisons, but also for the study of 

 the various specializations, which apparently were constantly taking 

 place in this family during the latter part of the Oligocene and through- 

 out the Miocene. 



In P. carrikeri it is clear that the shortening of the limbs went hand 

 in hand with the shortening of the skull, and it is presumable that 

 P. chelydra from the John Day showed the same tendency to a marked 

 degree. On the other hand in P. gmwrZi^, in which the skull and the bony 

 frame-work in general are lighter, the limbs are longer and slenderer. 



The skeletons of P. carrikeri in the positions in which they were 

 originally found furnish a clue to the habits of the animals as well as 

 to the manner in which in this instance they were finally imbedded. 



1 Leidy, J., Proc. Acad. Nat. Sci. Phila., 1870, p. iii {Oreodon superbus). 

 U. S. Geol. Surv.. Vol. I, 1873, pp. 211-215; PI. I, Fig. i; PI. II, Fig. 16; PI. 

 VII, Figs. 7, 8, 9, 10, II, {Oreodon superbus). Bettany, G. T., Quart. Jour. 

 Geol. Sac, Vol. XXXII, 1876, pp. 259-273, {Merycochoerus leidyi). Cope, E. D., 

 Proc. Amer. Philos. Soc, Vol. XXI, 1884, pp. 503-572. 



2 "Mammalia of the Deep River Beds," Trans. Amer. Philos. Soc, \'ol. X\'II, 

 1893. PP- 151-162. 



