148 Proceedings of the Biological Society of Washington. 



luteolus and other small mammals inhabiting southwestern Wyoming, 

 suggests that all have been similarly affected by local environmental con- 

 ditions. Specimens from Bear River, near the Utah border, are compara- 

 tively dark in color and apparently grade toward P. p. olivaceus. Those 

 from southeastern Idaho are somewhat intermediate, as they combine the 

 color of typical P. p. clarus with the smaller average size and more 

 slender skull of P. p. parvus which inhabits the Snake River Valley in 

 southwestern Idaho. 



Specimens examined. — Total number, 34, as follows: 



Idaho: American Falls, 8; Big Butte, 1; Birch Creek, 3; Blackfoot, 2; 

 Dickey, 2; Lemhi, 1; Pahsimeroi River, 1; Pahsimeroi Valley, 2. 



Wyoming: Bear River (14 miles north of Evanston), 2; Cumberland, 9 

 (type and topotypes); Fort Bridger, 1 ; Mountainview, 2. 



Perognathus flavus piperi, subsp. nov. 



Type from 23 miles southwest of Newcastle, Wyoming. No. 168,650, c? 

 adult, U. S. National Museum (Biological Survey Collection) ; collected 

 by S. E. Piper, May 25, 1910. Original number 283. 



Geographic distribution. Known only from the type locality, but 

 probably ranging at low elevations throughout eastern Wyoming and 

 western South Dakota. 



General characters. — Most closely allied to Perognathus flavus flavus 

 but larger; upperparts less intense ochraceous buffy; skull larger and 

 differing in detail. Similar to P. f. bimaculatus, but smaller and paler 

 colored. 



Color. — Upperparts between light buff and light ochraceous- buff, finely 

 mixed or overlaid with black ; lateral line and postauricular spots pale 

 but distinct; feet and underparts white as usual in the group; tail whit- 

 ish all around. 



Skull. — Essentially like that of P. f. flavus in general form, but larger 

 throughout; nasals longer, reaching anterior plane of orbits (not normally 

 reaching this plane in typical flavus); mastoids decidedly larger; inter- 

 parietal quadrate in outline, the posterior border more deeply emarginate. 

 Differing from that of P. f. bimaculatus in decidedly smaller size. 



Measurements. — Type: Total length, 113; tail vertebrae, 51; hind foot, 

 17. Two adult topotypes, respectively: 116, 115; 49, 50; 17, 17. Skull 

 (type) : Greatest length, 22; greatest mastoid breadth, 12.4; interorbital 

 breadth, 4.6; nasals, 8.1; width of interparietal, 2.7; length of inter- 

 parietal on median line (between anterior border and posterior emargi- 

 nation), 1.6; maxillary toothrow, 3.4. 



Remarks. — Perognathus flavus piperi is readily distinguished by the 

 combination of color and cranial characters pointed out. Its geographic 

 range marks the northern limit of the species, which on the south reaches 

 the Valley of Mexico. Specimens from eastern Colorado are assignable 

 to P. f. flavus, but in less buffy coloration apparently grade toward the 

 present form. 



Specimens examined. — Three, all from the type locality. 



