98 THE MAMMALIA OF THE DEEP RIVER BEDS. 



The mandible has a long, stout and rather deep horizontal ramus, which tapers 

 less anteriorly than in the White River form. The symphyseal portion is long and 

 procumbent, quite sharply constricted at the diastema and expanding again to form 

 the semicircular alveolus in which the incisors and canines are inserted. In this por- 

 tion of the mandible, this species resembles Mesoliippus more than JEquus ; it differs 

 from both in the rapid rise of the inferior border towards the chin. The ascending 

 ramus is also intermediate in character between the same genera. "With the former 

 it agrees in the shape of the angle, which is set off from the posterior border by a 

 notch some distance below the condyle; in the broad anterior border of the ascend- 

 ing ramus, with its deep groove for the buccinator and maxillo-labial muscles and 

 prominent linea obliqua externa. The latter gradually approximates the linea interna 

 and unites with it to form the anterior border of the coronoid. On the other hand, 

 the ascending ramus is decidedly higher than in Mesoliippus and the condyle is 

 greatly elevated above the level of the molars, in correlation with the raising of the 

 base of the cranium already referred to. This elevation is, however, much less in 

 proportionate amount than occurs in the horse, where the vertical height of the 

 ascending ramus, measured to the condyle, is more than half the length of the hori- 

 zontal ramus (about 5 : 8). The coronoid is much better developed than in either the 

 lower Miocene or the recent genus ; it is very high, erect and compressed ; the free 

 end is somewhat flattened obliquely and recurved, and the posterior border is nearly 

 straight. The coronoid notch is narrower and deeper than in the White River 

 species, and the condyle more extended transversely, especially towards the exter- 

 nal side. 



Measurements. 



M. 



Mandible, length 305 



Mandible, height of condyle 126 



Mandible, height of coronoid from sigmoid notch 052 



Mandible, depth at pTl 030 



Mandible, depth at m. 3 052 



Lower median incisor, width 010 



Lower second incisor, width 009 



Lower third incisor, width 007 



Lower molar-premolar series, length 148 



Lower premolar series, length 078 



Lower first premolar, length 013 



Lower first premolar, width 006 



Lower second premolar, length 021 



Lower second premolar, width 009 



Lower third premolar, length 022 



Lower third premolar, width 013 



