THE MAMMALIA OF THE BEEP EIVER BEDS. 95 



that will be explained in the full description; (8) the median digit is more enlarged 

 and its ungual phalanx shorter and more rounded, but also flatter and more depressed. 

 The type specimen of the species consists of a fragmentary skull (with the den- 

 tition almost complete), several vertebras from different regions, the fore limb (lack- 

 ing the scapula) and the pelvis. Several other fragmentary specimens are referable 

 to the same species. 



Dentition. A. Upper Jaw (PI. Ill, Fig. 24). The incisors decrease regularly 

 in their dimensions from the first to the third; they have very short, but broad and 

 thick crowns, and already present a decidedly horselike appearance ; the cingulum is 

 elevated, and thus between this structure and the front margin of the crown a pit is 

 formed. In spite of the fact that this genus, as will be shown in the sequel, is almost 

 certainly not in the direct line of equine descent, we may conclude with great proba- 

 bility that these teeth explain the genesis of the invagination in the incisors of the 

 recent horses and that, in the latter, the hind wall of the pit is to be regarded as a 

 greatly enlarged cingulum. The canine has been lost, but the alveolus shows that it 

 was rather small and separated from the incisors by a short diastema and from pJ by 

 a longer one. 



The first premolar, as in the European species, is relatively larger than in the 

 more ancient genera of the phylum, e. g., Mesoliippus. On the outer side it is con- 

 vex and so obscurely divided that a tritocone can hardly be said to be present ; the 

 deuterocone is a long, low ridge, ending posteriorly in a cone, which probably repre- 

 sents the tetartocone in an incipient stage. As in the horses generally, the second 

 premolar is the longest tooth in the series. This elongation from before backward is 

 due to the enlargement of the antero-external buttress, or protostyle, which, however, 

 is less extreme than in A. aurelianense ; it is separated from the protocone by a fold 

 or ridge of enamel. This tooth differs further from the succeeding ones in the 

 greater narrowness of its anterior portion, which produces a shortening of the ante- 

 rior transverse crest, and the posterior is slightly separated from the external wall. 

 The third and fourth premolars differ comparatively little from the corresponding 

 teeth in the European species ; the external crescents are not quite so deeply con- 

 cave, the outer cingulum is somewhat more, and the inner somewhat less developed. 

 The posterior pillar is large and on wear becomes connected with the posterior crest, 

 so that the hinder valley is completely enclosed. 



The upper molars are likewise very similar to those of A. aurelianense, except 

 that the transverse crests are somewhat straighter and the conules even more reduced. 

 The third molar is much the smallest of the series and differs quite markedly from 

 the corresponding tooth of the European species. The posterior crest is not curved, 



A. P. S. — VOL. XVIII. M. 



