THE MAMMALIA OF THE DEEP RIVER BEDS. 97 



the skull (PI. Ill, Fig. 23). So far as can be judged from the portions preserved, it 

 very much resembles that of the White River MesoJiippus and displays comparatively 

 little modernization. The face has become somewhat lengthened by the recession of 

 the orbits, but not so much as in the J ohnDay MioMppus pra&stans ; the anterior rim 

 is over m. g , while in MesoJiippus it is over m. 1 . The infraorbital foramen has accom- 

 panied this recession, so that its position with reference to the orbit remains the same 

 as before. In this species it is above M, in MesoJiippus over p-js. The orbit remains 

 very low down in the face, or rather has descended still lower relatively, owing to 

 the development of large frontal sinuses, while the molars remaining very short 

 crowned give no occasion for elevating the orbits. Consequently, the supraorbital 

 region has a much greater vertical depth than in the "White River genus. The orbit 

 is still open behind, but the postorbital process is somewhat longer and more recurved. 

 The supraorbital foramen of Equus is, as in the older genus, represented by a deep 

 notch, but the spine is more prominent and nearer to the postorbital process, which 

 appears to be a step towards converting the notch into a foramen. !Not enough of 

 the lachrymal is preserved to determine whether a lachrymal pit is present or not. 

 The cranium is much more elevated above the level of the face than in MesoJiippus, 

 and this results in giving the zygomatic arch a much more abrupt descent downward 

 and forward. The zygomatic process of the squamosal is rather high vertically, but 

 very thin and compressed. The glenoid cavity has the characteristic equine appear- 

 ance even more decidedly marked than in the more ancient form and its outer portion 

 is raised into quite a prominent tubercle. The postglenoid process is very largely 

 developed and is much longer, heavier and more rugose than in JHquus. The eleva- 

 tion of the cranium, unaccompanied by a corresponding rise in the position of the 

 orbits, brings about a curious combination of primitive and advanced characters, 

 a combination which may, for the most part, be referred to the elevation of the 

 cranium together with the retention of the primitive braelryodont dentition. The 

 premaxillary region is decidedly more equine than in MesoJiippus. Corresponding to 

 the increase in the relative dimensions of the incisors, the horizontal portion of the 

 premaxillaries is more largely developed, especially in the vertical direction. The 

 symphysis is quite high and ends above in an obtuse spine, and the ascending ramus 

 makes a more decided angle with the horizontal part. Compared with the premax- 

 illary of the horse, the chief difference to be observed is the rapid descent of the 

 upper margin of the bone at a point above the diastema between the incisors and 

 canine, so that at this point the vertical height is much less than elsewhere. This 

 feature does not occur in either Equus or MesoJiippus. There is no such constriction 

 of the muzzle behind the canine as is seen in the latter genus. 



