152 THE MAMMALIA OF THE DEEP RIVER BEDS. 



" the zygoma as far as the anterior border of the glenoid cavity is slender, and not 

 convex, but flat in every direction, nor is it decurved, as in M. superbus. The zygo- 

 matic foramen is relatively much smaller than in that species, but is oblique outwards 

 and forwards at an open angle. The obtuse median edge of the zygoma looks 

 upwards, not outwards, as it does in M. superbus and M. macrostegus, and the supe- 

 rior expansion is opposite the internal extremity of the glenoid face, instead of the 

 external, as in M. superbus, or the middle, as in M. macrostegus" (Cope, ISTo. 7, 

 p. 531). 



None of the specimens in the Princeton collection agree altogether with the type 

 in the construction of this region of the cranium. The description applies best to an 

 almost perfect skull, which, however, differs from the. type in the outward and back- 

 ward direction of the posterior or preglenoid boundary of the "zygomatic foramen." 

 The shape of this opening is different from that seen in the two John Day species 

 which have been mentioned in having its longer axis directed antero-posteriorly, 

 while in them it is transverse. In this specimen the massive and rugose superior 

 expansion of the zygoma is, as in M. macrostegus, above the middle of the glenoid 

 cavity, not above its internal edge, as in the type of the species. A second specimen 

 has the long axis of the zygomatic opening transversely directed, as in the John 

 Day species, and the zygomatic process with its superior expansion is almost as 

 heavy and rugose as in M. macrostegus. This specimen may perhaps represent 

 another species, but the variation in this region of the skull is so great that species 

 can be distinguished only with difficulty. Other specimens show differences of 

 greater or less degree from the two which have been described, which increases the 

 difficulty of distinguishing species. 



The jugal is flattened and beneath the orbit has great vertical depth. The 

 orbits present more laterally and less obliquely forward than in M. macrostegus ; they 

 are also smaller and less prominent than in that species. The forehead is much nar- 

 rower than in the John Day form and made more convex by the enlargement of the 

 frontal sinuses and is decurved at the orbits. In M. macrostegus this decurvature 

 does not occur ; the forehead is very wide and almost perfectly flat. The lachrymal 

 is very large and has a deep pit which is very much better marked than in M. ma- 

 crostegus. The facial portion of the maxillary is shorter but has a greater vertical 

 height. The nasals are very long and project beyond the alveolar border of the pre- 

 maxillaries, as they apparently do not in M. macrostegus ; their free ends are obtusely 

 rounded and slightly decurved. They are broader and less decurved at the edges 

 than in the John Day species, but nevertheless are somewhat convex from side to 

 side. The shape of the muzzle is very different in the two species; in the more 



