63 



part of a lower jaw, including both rami, obtained by Dr. Carter, at Bridger 

 Butte, seven miles west of Fort Bridger. The left ramus contains the pos- 

 terior three premolars and the succeeding two molars, of which a view of the 

 triturating surfaces is given in Fig. 26. 



The lower molars, including the last one, in Hyrachyus agrarius have all 

 bi-lobed crowns. These are oblong square, and bounded by a basal ridge in 

 front, behind, and in a more or less interrupted condition externally. The 

 constituent lobes have somewhat curved rectangular summits as in Hyracodon 

 and rhinoceros. The summit of the anterior lobe curves forward and inward, 

 and becomes continuous with the basal ridge of the fore part of the crown. 

 As the acute summits are worn, tracts of dentine become exposed 

 crossing the teeth. In the progress of attrition the expanding dentinal tracts 

 extend in an irregular L-like manner, and finally the contiguous tracts of each 

 tooth become continuous, as in rhinoceros at the same stage of wear. 



The crowns of the premolars present the same essential constitution as 

 those of the molars, with the constituent lobes, successively, from behind 

 forward, becoming more reduced or rudimcntal. The posterior lobe becomes 

 proportionately more reduced than the anterior, and in the first premolar has 

 disappeared. 



The crown of the last premolar resembles those of the molars, with the 

 posterior lobe proportionately more reduced than the anterior one. 



The crown of the third premolar, in the specimen represented in Fig. 25, 

 Plate XX, has the same form as in the last premolar, and is simply reduced 

 in size. In the specimen represented in Fig. 13, Plate IV, the tooth looks 

 different, from the oblique ridge or summit of the anterior lobe of the crown 

 as existing in the former, being contracted in this into a conical and some- 

 what more elevated point. This gives such a remarkable difference to these 

 teeth in the two specimens, that, had they been found isolated, without a 

 knowledge of their collocation, they would have been attributed to different 

 genera of animals. 



The anterior two premolars have an oval crown elevated into a median 

 conical point and presenting offsets behind and in front, in which may be 

 detected the rudiments of the posterior lobe and anterior extension of the 

 anterior lobe of the better developed crowns of the teeth behind. 



All the lower premolars, as well as the molars, are inserted by a pair of 

 fangs. The space occupied by the lower molar series in several specimens 

 ranges from 3 inches and 5 lines to 3if inches. 



