Article XXII.— A HORNED RODENT FROM THE COLO- 

 RADO MIOCENE. WITH A REVISION OF THE 

 MYLAGAULI, BEAVERS, AND HARES OF THE 

 AMERICAN TERTIARY. 



By W. D. Matthew. 

 Ceratogaulus rhinocerus, n. g. et sp. 



The writer has recently described part of the skull of a 

 Mylagaulus from the Colorado Loup Fork beds, found in 

 1898. A nearly complete skull, with one ramus of the lower 

 jaw, found by Mr. Brown of the Expedition of 1901, indicates 

 a new genus of this family, distinguished by the unique 

 character (for a rodent) of a pair of large connate processes on 

 the nasals resembling the horn-cores of some Ungulata, and 

 giving the skull a prcJfile absurdly like that of a miniature 

 rhinoceros. 



The skull is a little larger than that of Mylagaulus, and dis- 

 plays considerable modifications, chiefly conditioned by the 

 development of the horn-like processes on the nasals. The 

 muzzle is much wider and tapers forward; the nasals are 

 much wider throughout, and especially in the middle, where 

 they bear the horn-cores. The postorbital processes of the 

 frontal and jugal bones are considerably less prominent and 

 placed farther back, making the orbit larger and more ex- 

 tended anteroposteriorly. The zygomata are deeper. The 

 enlarged molar in the upper jaw differs a little in form, and 

 considerably in the pattern of the crown. The penultimate 

 upper molar appears to be considerably larger in proportion, 

 but is so much damaged in the Mylagaulus skull that it cannot 

 be closely compared. The enlarged molar of the lower jaw 

 displays a crown pattern with the usual lakes in three longi- 

 tudinal rows, instead of four as in Mylagaulus. The alveoli 

 of the second and third molars are of nearly equal size, while 

 in Mylagauhis the penultimate alveolus is much larger. The 

 type specimen No. 9456, is of nearly the same age as the My- 

 lagaulus skull with which it has been compared, the wear of 



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