THE MYLAGAULID€; AN EXTINCT FAMILY OF SCIUROMORPH 
RODENTS. 
The genus My/agaulus*, 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, JZ. Monodon and M. sesquipedalis. Water the fam- 
ily name, MZy/agaulide, was proposed for these forms. To this 
group it is now proposed to add the two genera described below. 



——__ 
Dentition of same as seen from above X 8. 
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, itsnearest affinities are with W/y/agaulus, Cope, though it is 
amore 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 My/agaulus 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.t 
*Bulletin of the U.S. Geological and Geographical Survey, vol. iv., No. 2, May, 1878. 
tAmerican Naturalist, Jan. 1883. 
+The Mammalia of the Deep River Beds, Am. Phil. Soc., Oct. 1893. 
181 be . 
