No. 2.] THE RELATIONS OF PROTOCERAS. 309 



The fourth milk molar is molariform and differs from the 

 permanent ones only in size. 



II. The Skull (PI. XXI, Figs. 1-3). 



No genus hitherto known from the White River formation 

 has such a modernized type of skull as Protoceras, which, in- 

 deed, is in some respects in advance of the existing hornless 

 deer, such as MoscJms and Hydropotes, and in one or two points 

 exceeds all the CervidcB, approaching to the highest type of 

 artiodactyl structure, the Cavicornia. At the same time, 

 remnants of the primitive condition are by no means wanting, 

 and the resulting combination is a very curious one. Another 

 characteristic feature of the skull in Protoceras is the extreme 

 degree to which the sexual differences are carried. Such an 

 amount of sexual difference is altogether unparalleled among 

 the ancient artiodactyls and is not attained even among the 

 modern Ccrvidce. The modernization of the skull, which has 

 been referred to, consists in the following characters : (i) the 

 shortening and rounding of the cranium ; (2) the backward 

 shifting of the orbit, which is removed entirely behind the line 

 of the molar teeth ; (3) the great elongation of the facial region, 

 due not only to the shifting of the orbit, but also to the length- 

 ening of the muzzle ; (4) the bending downward of the face 

 upon the cranio-facial axis, which does not occur in the CervidcB. 



The material at command displays three well-marked types 

 of skull-structure, two of which are undoubtedly due to sexual 

 distinctions, while the significance of the third is not yet 

 entirely clear. It will be most convenient to begin the 

 description with an account of the unmistakably female skulls, 

 since these are the less extremely specialized type, and there- 

 fore best fitted to display the fundamental characters of struc- 

 ture. Of these females, there are five finely preserved skulls 

 in the Princeton collection, with fragments of several others. 

 The females are much more abundant than the males, as 

 would be expected from the analogy of recent ruminants. 

 Among these skulls there is a considerable degree of variation, 

 and perhaps more than one species is represented by the 



