224 University of California Publications in Zoology [Vol.12 



Citellus tereticaudus tereticaudus (Baird) 

 Round-tailed Ground Squirrel 



Fairly common at intervals from Needles to the Mexican line. Six- 

 teen specimens were collected (nos. 10637-10651, 10722) on both sides 

 of the river, as follows : California side : five miles below Needles 1, 

 opposite Cibola 2, near Pilot Knob 4 ; Arizona side : at Mellen 5, five 

 miles northeast of Laguna 3. The species was seen at no other places, 

 and the inference is to be drawn that it is of quite interrupted distri- 

 bution. This is explained when its associational position is taken 

 account of. 



Citellus tereticaudus is a sand-dwelling mammal, resembling in 

 its preferences Dipodomys deserti, Peromyscus eremicus and Perog- 

 nathus penicillatus. The squirrel, however, is even more restricted 

 than either of these, for a rather large extent of arenaceous territory 

 seems to be necessary to support a representation. Where the second 

 bottom was broad, with accumulations of wind-drifted sand about the 

 scattering Atriplex and creosote bushes, tereticaudus was best repre- 

 sented. Such a place was afforded on the Arizona side above Mellen. 

 On the desert mesa similar conditions were rare, but where they did 

 occur to any extent, as near Pilot Knob, the round-tailed ground 

 squirrels were usually in evidence. The burrows open up among the 

 stems of partly buried bushes, the animals foraging out over the bare 

 intervals where seeds from annual plants are found sifted into the 

 surface sand. 



No young were seen by our party; but two females taken opposite 

 Cibola, April 3 and 4, contained six and four embryos respectively. 

 There are in the Museum two half-grown young (no. 7767, 7768) 

 taken by Charles L. Camp at Needles, July 15 and 19, 1909. 



The series of Citellus tereticaudus, when segregated into two groups 

 according to the side of the river from which the individuals were 

 obtained, shows appreciable diversity in external features. The speci- 

 mens from the Arizona side are distinctly darker, more brownish than 

 those from the opposite (California) side, even where obtained within 

 but a very few miles of one another. This diversity in color is most 

 marked at Needles, less so lower down the river. It would appear 

 that the case is quite comparable to that in Perognathus penicillatus, 

 where the darkest individuals hail from the east side of the river north 

 of The Needles. 



