216 M. R. Thorpe — John Day Pronierycochoeri, 



a small style developed from the liypocone, of which it 

 is an integral part. 



There is also great variation in the size, shape, and 

 curvature of the zygomatic process, including the malar 

 below the orbit. These variations do not correlate with 

 the size of the canine, and do not appear to be a sexual 

 character, as Scott^ pointed out, and as studies of the 

 present material likewise show. The development of the 

 zygoma in this group is so remarkable as well as unusual 

 that it must possess considerable significance. The chief 

 function was apparently to furnish sufficient surface for 

 attachment for the powerful muscles necessary in the 

 mastication of the coarse food upon which these animals 

 subsisted. It has been suggested that these processes 

 may indicate the presence of some external embellishment, 

 as for example, the excrescences on the African wart-hog. 



There is likewise a very marked variation in the various 

 skull elements, while the various parts of the skeleton 

 have changed but very little, except in size, from those of 

 the earlier Eporeodons. This variation is due, probably, 

 to some external causes which are reflected in the skull, 

 where the greatest evolution is localized in all of the 

 genera of the Oreodontidas (Merycoidodontidae). The 

 John Day representatives of the genus Pr ornery cochoerus 

 became extinct, so far as now known, with the close of the 

 Oligocene. To what causes these variations and extinc- 

 tion are due, is not clear. Possibly a changing climate 

 with its concomitant floral changes was responsible, or a 

 different environment caused by the former habitat 

 becoming uninhabitable through the influx of poisonous 

 gases, or ash falls occurring with sufficient frequency and 

 volume to render the area barren and devoid of life. 

 Racial old age or emigration may have been contributing 

 factors. The chief consideration is that this group of 

 large animals became extinct and that, before extinction, 

 they had begun to vary to a great degree. Many other 

 groups also became extinct in this basin with the end of 

 Oligocene times, such as Eporeodon, Agriochoerus, Meso- 

 hippus, Protapirus, Elotherium, and many of the carni- 

 vores and rodents. 



In connection with the present study of Promeryco- 

 choerus, the writer wishes to express his appreciation of 

 the courtesy shown him by Messrs. Matthew and Granger," 



2 W. B. Scott, Trans. Amer. Philos. Soc, 17, 151, 1893. 



