1914] Grinnell: Mammals and Birds of the Colorado Valley 239 



Thomomys chrysonotus Grinnell 

 Ehrenberg Pocket Gopher 

 The only place on the Arizona side of the river where gopher sign 

 was seen was on the mesa within two miles back from Ehrenberg. Here 

 two sets of mounds were found on low sandy ridges and a single gopher 

 caught from a burrow, March 27. This specimen is a male, young adult 

 (no. 10617) and proved so different from any previously described 

 race of gopher as to warrant making it the basis of a new specific 

 name (see Grinnell, 1912. p. 174). This is evidently an upland species, 

 that is, not riparian, and its nearest relationships are suggested to 

 be with forms to the east and north rather than with T. perpallidus 

 or T. albatus of the desert region west of the river. 



Thomomys albatus Grinnell 

 Imperial Valley Pocket Gopher 



At only one station on the California side of the river did we 

 find any sign of gophers. This was at our last camp, east of Pilot 

 Knob, where on the site of the old Hanlon Ranch several sets of work- 

 ings were located, and eight specimens taken (nos. 10618-10625), May 

 7 to 10. Four of the animals were young, not more than half grown. 



These workings were all on the first bottom just at the outer mar- 

 gin of the arrowweed association, in ground barely reached at the 

 highest level of overflow. The absence of gophers in the greater por- 

 tion of the Colorado bottom is reasonably explained by the occurrence 

 of the yearly overflow which would drown them out. The colonies 

 invading the river bottom do so only at points which they reach from 

 some more favorable center of distribution back from the river. 



As already shown (Grinnell, 1912, p. 172), our gophers from the 

 Pilot Knob station belong to the species occupying the alluvial delta 

 region including Imperial Valley, which species is distinct from both 

 the one represented on the Arizona side at Ehrenberg (Thomomys 

 chrysonotus) and the one of the western end of the Colorado desert, 

 at Palm Springs {Thomomys perpallidus). 



The characters distinguishing albatus from chrysonotus cannot 

 be reasonably considered as due alone to the action of the river as a 

 barrier, for the former species belongs to a different association 

 (saltbush) from the latter which, as far as known, adheres to the 



