106 Proceedings of the Biological Society of Washington. 



mixed black and buffy hairs, becoming nearly pure black toward tip, pale 

 buffy below. 



Skull. — Essentially like those of P. p. parvus and P. p. olivaceus, but 

 larger than is typical of either, and decidedly broader, especially across 

 mastoid bullae. 



Measurements (type). — Total length, 189; tail vertebrae, 102; hind foot, 

 24. An adult male topotype, 184; 96; 24. Skull (type): Greatest length, 

 27.5; greatest mastoid breadth, 14.5; interorbital breadth, 5.9; length of 

 nasals, 10.4; greatest width of interparietal, 5.9; maxillary toothrow, 4. 



Remarks. — The remarkably dark color of P. p. idahoensis, distinctive 

 at a glance, appears to be associated with that of its lava field environment. 

 Additional specimens from the general region of the type locality are likely 

 to reveal a rather extensive range in the unexplored Snake River desert 

 country of southern Idaho. Specimens from southeastern Oregon, assigned 

 to P. p. parvus, are rather dark in color and indicate probable intergrada- 

 tion. In dark coloration the form here described contrasts strongly with 

 its pale geographic neighbor, P. p. darns, of eastern Idaho and southwestern 

 Wyoming. 



Specimens examined. — Two, from the type locality. 



