Nasals and horns of Mcgacerops syceras, fig. 9 viewed from above, fig. 10 from the front, fig. 11 from the right side. One- 

 fourth the natural size ; figs. 10 and 11, after Cope. 



Megacerops primitivus, Sp. nov. 



Plate VI, figs. 4 and 5. 



In 1904 the writer was fortunate in securing, from the Oligocene deposits of the Cypress 

 hills, a mandible of a Titanothere that is of considerable interest. It is the most perfect 

 lower jaw obtained so far from this locality, and is the type of the above named species. 



The specimen consists of both halves of the jaw. The left ramus is perfect, with the full 

 number of teeth preserved ; the right ramus lacks the fourth premolar, and the molars, with 

 that part of the alveolar border that held these teeth. 



The mandible is of fair size, and belonged to an adult individual. It presents the follow- 

 ing dental formula : Ij, C r , P 4 , Mj. There is a diastema behind the canine, a character 

 which, taken in conjunction with the presence of three incisors and four premolars, is 

 indicative of a somewhat primitive species. The chief characters displayed are : incisors, in 

 three pairs, with a space between the inner pair; canines, of small diameter, apparently short ; 

 a diastema between the canine and the first premolar ; first premolar, small ; third premolar, 

 becoming molariform, fourth premolar, molariform ; symphysis, long ; symphysial surface 

 between canines, narrow ; jaw contracted at the diastema ; external cingula, moderately 

 developed ; internal cingula, wanting ; mental foramen, beneath the second premolar; coronoid 

 process, short. 



Megacerops avus (Marsh), from the Oligocene of South Dakota, has three pairs of inferior 

 incisors but only three premolars below on each side, and there is a short diastema behind 

 the lower canine. Its dimensions are greater than those of M. primitivus. These two 

 species are apparently the only ones of the Oligocene Titanotheres in which there are three 

 pairs of incisors in the lower jaw. 



In the Cypress Hills specimen the crowns of the incisors are of a depressed spherical 

 shape, with a tendency to come to a rounded central point above. The second incisor is the 

 largest, and the first is slightly smaller than the third, which is the most upright. The first 

 is more procumbent than the second. Between the inner pair is a very decided interval, 

 leaving a space of 6 - 5 mm. between the crowns of the two teeth The crowns of the canines 



12529—7 



