60 ON THE EXTINCT MAMMALIA OF 



backward curvature, in front of the upper canines. Those, in the only skull in which 

 they are retained, are imperfect. An isolated specimen is represented in figures 4, 5, 

 plate xviii, of the Ancient Fauna of Nebraska. It has a curved conical crown 

 flattened behind, and with the posterior smaller surface defined by acute borders. 

 The j)oint of the inferior canine was received in the angular interval between the 

 base of the crown of the upper canine and that of the upper lateral incisor. 



Incisors. — The superior incisor teeth of the two sides together form a semi-circle 

 occupying the interval of the upper canines. They are proportionately much longer 

 than in the cats, and are isolated from one another and the contiguous canines by 

 well-marked intervals increasing successively from the first to the last. They also 

 successively increase in size from first to last. Their crown is curved, conical and 

 pointed, flattened behind, and have the posterior surface defined by acute borders, 

 as in the case of the inferior canines. Their base posteriorly appears somewhat 

 widened, but not so as to form a conspicuous heel as in the cats. 



The inferior incisors occupy the interval of the lower canines, without a hiatus 

 separating them from the latter. They form a slightly curved row, and are more 

 closely situated than the superior. They are smaller than those above, but like them 

 successively increase in size. They are broken in the only specimen of a skull in 

 which they are partially preserved, but appear to have had the same form as the 

 upper ones. 



The superior incisors curve foi'ward and downward; the inferior ascend almost 

 vertically, with a slight backward curvature. The points of the crowns of the latter 

 occupy the angular intervals of the crowns of the former, when the jaws are closed. 



The specimen of a skull o^ Drepanodon primcevus, described in the Ancient Fauna 

 of Nebraska under the name of Machairodus primceviis, and represented in plate xviii 

 of that work, was obtained during a geological survey of Dr. Owen, and is now pre- 

 served in the Museum of the Smithsonian Institution. It belonged to an adult 

 individual, and is intermediate in size to the two skulls represented in plate IV of the 

 present memoir. 



The specimen represented in figure 1 of the latter plate is the largest skull of D. 

 primaams which has been discovered. It was obtained during a geological survey of 

 Dr. John Evans, and was presented by the Smithsonian Institution to this Academy. 

 It is without the lower jaw ; has but part of the zygomata, the auditory bullas, and is 

 otherwise mutilated, but is comparatively well preserved. It belonged to an adult 

 individual. The upper sectorial molar is considerably worn ; and the upper first 

 premolars are lost and their alveoli obliterated. The frontal suture has also 

 disappeared. 



Tlie specimen represented in figure 5 of the same plate is an almost complete skull 



