830 THE JOHN DAY FAUNA. 



nifens, but the details differ from those of 31. hippodus in quite as great a 

 degree. Thus, the intermediate tubercles are lower and more conic, and 

 the external enamel border is not produced so as to inclose lakes, as in that 

 species. 



Other portions of the dentition are not determinable, except the superior 

 incisor. This is moderately compressed, and has a regularly convex ante- 

 rior face, without grooves. 



Measurements. 



M. 



Leogth of three large anterior molat teeth 0095 



„. . m o < anteroposterior 0036 



Diameters of Pm. 21 ^ „„., , 



( transverse UOdss 



Diameters of M. 2 j '*°'«°P°«t«''°' ^^0 



t transverse ..'. 0040 



Width of anterior face of superior incisor 0020 



Meniscomys cavatus Cope. 



BtJletin U. S. Geol. Surv. Terrs., VI, p. 366. 

 Plate LXIII; figs. 12-15. 



A concretion contains a number of limb bones and a mutilated skull of 

 this rodent. The latter displays the entire dentition of one side, in both jaws. 



Besides the characters of dentition which distinguish this species, the 

 ear-buUse differ from those of the M. Jiippodus. They are, as in that species, 

 very large and somewhat compressed, but they differ in being transversely 

 divided by bony laminae of no great depth, which give a sacculate appear- 

 ance like that of a colon, with the difference that the external surface is 

 not grooved at the bases of the partitions. The superior surface of the 

 skull is flat; there is no sagittal crest, and there is a well-marked postorbital 

 angle. The interorbital surface is plane. The occipital condyle is narrow, 

 and is separated from the otic bulla by a space as wide as itself The basi- 

 occipital is not excavated, but has a low median keel-angle. The superior 

 molar teeth are all posterior to the anterior border of the orbit, excepting 

 the anterior lobe of the second premolar. 



The anterior V of the second premolar is the largest; while on the 

 last molar the anterior V only is present, the place of the posterior one 

 being occupied by a flat space. There is a short external rib separating 

 the two external V-faces. On the grinding surface of the molars it is evi- 

 dent that the posterior intermediate tubercle is represented by two crenate 



