SCIUROMOREHA. 827 



and one internal crescentic sections of the investing enamel. On the sec- 

 ond superior molar there are three external crescents, and the first molar 

 is simply conic. Between the inner and external crescents there are the 

 curved edges of enamel plates directed obliquely and transversely. The 

 grinding surfaces of the inferior molars display, in the unworn condition, 

 curved transverse crests, connected longitudinally on the median line ; on 

 wearing, the lateral emarginations of the enamel become shallower, disap- 

 pearing from the inner side, but remaining on the outer. Incisor teeth not 

 grooved. Foramen infraorhitale anterius small, inferior, and near the orbit. 

 Postorbital processes; no sagittal crest. 



The characters of the dentition of this genus resemble those of the 

 genus Pteromys, which is now confined to Asia and the Malaysian Archi- 

 pelago. The superior molars differ from those of Pteromys in wanting all 

 reentrant enamel inflection. 



The general characters of the skeleton are unknown. A femur is rather 

 slender, and a tibia rather elongate, showing that the limbs are not short. 



Four species of this genus are known to me, all from the Truckee 

 Miocene of Oregon. They differ considerably in the details of the struct- 

 ure of the molar teeth. Their more prominent characters may be set forth 

 as follows : 



I. Superior molars short -rooted, with the external face plane; inferior molars with a 



prominent median transverse crest. 

 Smaller; dental crests fewer, simple, not crenate M. hippodus. 



II. Superior molars long- rooted; external face of crowns inflected, forming two V's; 



no median transverse crest on inferior molars. 



fl. Crests of superior molars fewer, simple, not crenate. 

 Larger; crowns short M. liolophus. 



aa. Crests of superior molars more numerous and much crenate. 



Smaller; plications of inferior molars shallow; borders raised M. cavafus. 



Larger; plications of inferior molars profound M. nitens. 



There is a suggestive resemblance between the forms of the molar 

 teeth of the Meniscomys hippodus and those of the Haplodontia rufa now 

 living in Oregon. The two genera have doubtless had a common origin, 

 but the present differences are considerable. Thus, the Haplodontia has an 

 extended osseous cavum tympani which does not exist iu Meniscomys. 



This genus appears to be referable to the Sciuridce. 



