2 14 Bulletin American Museum of Natural History. [Vol. XIX, 



preserved in the type of 5. ballovianus) is considerably wider 

 transversely, its anterior cingulum much less developed, its 

 mesostyle or median external cusp (between the anterior and 

 posterior transverse ridges) more prominent. In none of the 

 other species is the upper dentition known, so that an exact 

 comparison is not possible. 



Compared with modern Sciuridae this species shows some 

 interesting points of difference. It is nearest to Sciurus, but 

 differs in several points of importance: 



1. The cross crests on the molars are less complete, and are 

 partly broken up into separate cusps. 



2. The third premolar is a much larger tooth and has a 

 small accessory posterior cusp. 



3 . The heel of m3 bears a short transverse crest and a strong 

 posterior marginal ridge. In Sciurus the posterior part of 

 m3 is a slightly concave fiat basin. 



4. The base of the zygoma is anterior to p^ 



From Taniias and the other modern Sciuridae it also differs 

 in the retention of the mesostyle, in addition to most or all 

 of the above-mentioned points. The ridges are not so high 

 as in Cynomys and Arctomys, and their patterns differ in 

 various other details. I have little doubt that with more 

 perfect material it will be necessary to place this and all the 

 other Oligocene Sciuri in a separate genus, nearest to Sciurus, 

 but retaining the above primitive features in the dentition, 

 and others of more importance in the skull. But with our 

 present knowledge the distinction in the last molar is the only 

 one which we can predicate of all the Oligocene species in 

 common, and this is only of subgeneric importance at best. 

 The last lower molar in 5. relictus, ballovianus, and wortmaiii 

 exhibits a correspondingly ridged and unreduced heel to the 

 ridged and unreduced heel of the last upper molar in 5. vetustus. 



Upper dentition, p3-ni3 y_y 



molars m'-ni3 



mm. 



S-5 



P3 transverse 0.9 



longitudinal 0.8 



P4 transverse 2.3 



" longitudinal 2.0 



Mi transverse 2.5 



" longitudinal 1.8 



