292 University of Calif ornm Piiblications in Zoology [Vol. 21 



Remarks. — This race of the Microtus mordax group exhibits con- 

 siderable geographic variation throughout its range in California, 

 Specimens from the Trinity Mountain region and from the southern 

 part of its range in the Sierra Nevada are much darker than those 

 from intermediate localities. Similarly, specimens from the White 

 Mountains are much lighter than any from the nearby Sierra Nevada. 

 There can be little doubt, however, that there is an opportunity for 

 continuous dispersal along streams which empty into Owens River 

 from the Sierra Nevada and thence up similar streams which drain 

 the White Mountains. Sierrae is common throughout its range in 

 suitable places such as high mountain marshes and wet woods and, 

 more especially, along the banks of cold mountain streams. Indi- 

 viduals often descend these streams for a considerable distance below 

 the normal zonal position of the species. 



So far as is known sierrae is separated from hernardinus bj- a 

 wide interval. Also no specimens are known from the considerable 

 width of territory between the head of Rush Creek, Siskiyou County, 

 whence sierrae has been obtained, and the northwest coastal strip 

 occupied by angusticeps (see map, fig. CC) . There is no reason known 

 to the writer which would prevent the dispersal of sierrae down Rush 

 Creek to the Klamath River and thence to the seacoast ; future collect- 

 ing may supply the necessary material to establish the fact of inter- 

 gradation between sierrae and angusticeps. In fact large specimens 

 of mordax have been collected near Crescent City and Eureka which 

 are very perplexing and can only be explained on this assumption. 

 Elliot (1907, pp. 295-296) referred these large individuals to 

 Microtus toivnsendi, but they certainly belong to the mordax group. 

 In some respects these specimens are intermediate between typical 

 angusticeps and sierrae. 



