124: Troxell — Rodents of Genus Ischyromys. 



My sops, and Prosciurus, and comes to the conclusion that 

 the genus Ischyromys reached extinction in Oligocene 

 time. In the present paper, however, the suggestion is 

 offered that they may have developed into the modern 

 prairie-dog of the genus Cynomys. 



Description of New Species. 

 Ischyromys pliacus, sp. nov. 



(Fig. 1.) 



Holotype, Cat. No. 12511, Y. P. M. Middle Oligocene, Cherry Creek, 

 Colorado. 



Fig. 1. — Ischyromys pliacus, sp. nov. Lower right dentition of the largest 

 species of the genus. Note the many small cusps on teeth. Holotype. 

 Cat. No. 12511, Y. P. M. X 2 - 



This specimen, consisting of the right lower jaw, is 

 notable for its large size and for the great number " of 

 tubercles and deep pits on both premolar and molars. 

 The posterior cross crests do not arise directly from the 

 external tubercle but from its union with the central 

 longitudinal ridge, thus forming a "Y'\ This cross 

 crest is made up of two distinct minute cusps, but only on 

 P 4 and Mo are they still unworn. Small additional tuber- 

 cles are to be seen on the posterior side of the anterior 

 cross crests on all the teeth and conspicuous cusps occupy 

 the wide openings of the external grooves. 



Measurements are given in a table further on. 



Ischyromys typus nanus, subsp. nov. 



(Figs. 2, 3.) 



Holotype, Cat. No. 12519, Y. P. M. Oligocene (lower Oreodon beds), 

 Warbonnet ranch, 12 miles north of Harrison, Nebraska. Paratype, Cat. 

 No, 12555. Gerry 's Eanch, Weld Co., Colo. 



The specimens here described consist of the small lower 

 jaws with the molar teeth. The holotype was a part of a 

 young animal in which the last molar was just being cut. 

 Besides the fact that this is the smallest subspecies of 

 ischyromyds, it is further distinguished by the very nar- 



