THE MYLAGAULID/E; AN EXTINCT FAMILY OF SCIUROMORPH 



RODENTS. 



The genus Mylagaulus*, Cope, was originally based upon a sin- 

 gle lower premolar tooth, but later it was supported by the descrip- 

 tion of a fairly complete mandible, f From this material two species 

 were determined, M. Monodon and M. sesquipedalis. Later the fam- 

 ily name, Mylagau/idce, was proposed for these forms. To this 

 group it is now proposed to add the two genera described below. 



Lateral view of mandible of Mesogaulus ballensis X \. 



Dentition of same as seen from above X \. 



Mesogaulus ballensis, gen. et sp. nov- 



A rodent mandible from the Deep River escarpments near White 

 Sulphur Springs, Montana, and now in the Museum collection, shows 

 interesting characters hitherto undescribed. As suggested by Pro- 

 fessor Scott, its nearest affinities are with Mylagaulus, Cope, though it is 

 a more primitive form and evidently stands in an ancestral relation to 

 that group. The name Mesogaulus ballensis is proposed for it in 

 honor of Mr. S. H. Ball, the collector. The characters distinguish- 

 ing this genus from Mylagaulus are: First molar elongate antero-pos- 

 teriorly with enamel slightly inflected externally ; the third molar 

 rather stronger than the second and rotund in outline. The type 

 specimen was found in the alluvium at the base of an escarpment, so 

 that its horizon cannot be exactly determined. However, it certainly 

 belongs to the upper beds.^ 



•Bulletin of the U. S. Geological and Geographical Survey, vol. iv., No. 2, May, 1878. 

 tAmerican Naturalist, Jan. 1883. 



JThe Mammalia of the Deep River Beds. Am. Phil. Soc, Oct. 1893. 

 I8l 



