1914] Grinnell: Mammals and Birds of the Colorado Valley 267 



Pipistrellus hesperus hesperus (H. Allen) 

 Canon Bat 



The most abundant representative of the order Chiroptera 

 observed during the period of our work. Seen abroad at dusk as early 

 in the season as February 23, when the nights were still so cold thai 

 ice formed in suitable places. Numerous at Mellen, February 23 to 28, 

 and swarming in the vicinity of The Needles March 1 to 3. Thence- 

 forth seen at nearly every station all the way down the river. One 

 thing was conspicuously noticeable in regard to occurrence, namely, 

 that this bat varied directly in degree of abundance with nearness 

 to cliffs, or hillsides with outcroppings of fractured rock. In other 

 words, this species probably dwelt exclusively in the rocks during the 

 day. from which it emerged at early dusk to forage out over the river 

 bottom in the near vicinity. 



Individuals were often seen before the sunlight had yet left the 

 eastern hilltops. On one occasion, as we were floating down the river 

 near Picacho, a Pipistrellus appeared in flight in the glaring forenoon 

 sunshine, dipped down to the surface of the water, where it touched, 

 and thence flitted back to a crevice in the nearby cliff. 



Seventy-four specimens of this species were shot (nos. 10382-10423. 

 10746-10777), 42 being preserved as skins and 32 as alcoholics. 

 Eighteen out of this series are from the California side of the river 

 near Pilot Knob, only about six miles due west of old Fort Yuma, and 

 are thus practically topotypes of Pipistrellus hesperus. 



In reviewing the Museum's entire collection of Pipistrellus from 

 California, it becomes clearly apparent that while there is but one 

 species represented, there are two appreciably different subspecies of 

 this species, one a pale-colored form occupying the arid desert regions 

 from Owens Valley and the vicinity of Walker Pass southeast to 

 the Mexican line, the other a darker-colored form occurring on the 

 Pacific slope of southern California (in the San Diegan district) and 

 in certain parts of the San Joaquin and Sacramento valleys. 



It appears to the writer that we have here two races well worthy 

 of recognition in nomenclature. The name hesperus was based on the 

 desert form. A name is apparently available for the Pacific race in 

 the Vesperugo merriami of Dobson (1886, p. 124). This has always 

 been synonymized under Pipistrellus hesperus. While the habitat is 

 given by Dobson as "North America (Locust Grove, State of New 

 York) ", an error was committed, for the type really came from Red 

 Bluff, Tehama County, California (fide Miller, 1897, p. 31). Dobson 



