130 Troxell — Rodents of Genus Iscliyromys. 



The prairie dog now occupies the same Great Plains 

 region where once scurried the small ischyromyds, but 

 the conditions are greatly changed : the former is a bur- 

 rowing animal in a semi-arid climate; the latter lived 

 where the Oligocene streams from the young Rocky 



B 



i 



Fig. 7. — Iscliyromys typus lloydi, subsp. nov. Holotype. Cat. No. 12521, 

 Y. P. M. Nat. size. A, front and back views of humerus; note broad distal 

 end, supracondylar foramen on posterior side, spiral condylar grooves, and 

 large deltoid crest on one side. B, front and rear of distal end of the heavy 

 femur. C, right tibia viewed from inner side, showing deep double groove 

 for tendons to the foot. (See also figs. 4 and 5.) 



Mountains spread widely their flood waters, just as the 

 rivers from the east slope of the Andes do to-day. This 

 probably necessitated burrowing in the dry seasons and 

 at other times called for the ability to cope with the rising 

 flood. 



