Troxell — Rodents of Genus Ischyromys. 127 



the outer surface, this mandible resembles that of 

 Sciurus, cf. S. carolinensis. 



Additional distinctive features in the present sub- 

 species may be noted in the vascular impressions on the 

 supra-occipital and the slight overhanging of this bone, 

 the large otic bullae, slightly crushed down, the flat tri- 

 angle formed by the sagittal crest where it joins the inion, 

 and the absence of a deep groove on the zygomatic pedicle 

 near the maxillary. 



Fig. 5. — Ischyromys typus Jloydi, subsp. nov. Holotype. Cat. No. 12521, 

 Y. P. M. Side view of skull and jaws. Premolars just coming into view. 

 Skull slightly bent downward by crushing. Nat. size. 



Measurements of Holotypes. 



I. nanus Eatio I. pliacus 



mm. mm. 



Length of three lower molars 9.8 124 12.2 



Length of M t 3.2 116 3.7 



Width of M 1 .\ . . •. 2.7 137 3.7 



Length of M, 3.2 125 4.0 



Width of M, 2.7 152 4.1 



Length of M 3 3.2 131 4.2 



Width of M 3 3.0 123 3.7 



Ratio 



I. lloydi 





mm. 



113 



11.1 



113 



3.6 



119 



3.2 



116 



3.7 



126 



3.4 



119 



3.8 



110 



3.3 



Interpretation op Ischyromys. 



Wear of the teeth. — The long transverse ridges or 

 crests and the corresponding grooves led Leidy to observe 

 that the upper teeth were like the lower ones reversed; 

 this is true, however, only in a general sense. In addition 

 to the transverse crests of enamel, the teeth show series 



