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University of California Publications in Zoology [Vol. 12 



taken at Chemehuevis Valley. March 11, contained five embryos. It 

 is notable that the weather on the first date was very eold for the 

 region. It seems to be a rule among the Sciuridae of the Colorado 

 desert area that the young-of-t he-year are well grown long before the 

 period of intense summer heat. 



Ammospermophilus 1. leucwrus and A. h. harrisi are perfectly 

 distinct but closely related species. The following comparative table 

 serves to show the differences between them, which differences must 

 not be overemphasized as contrasted with the close general resemb- 

 lances. 



Ammospermophilus l. leucurus 



General size slightly less; body 



149.4 mm. 

 Tail relatively shorter, 44 per cent of 



body. 

 Audita] bullae slightly smaller. 



External ear slightly larger, 7.1 mm 

 General coloration paler above, less 



brown more gray. 

 White side-stripes more conspicuous. 



Tail beneath pure white. 



Tail carried appressed over the rump, 

 displaying under surface conspicu- 

 ously as if there were a white 

 rump patch. 



Tail often rapidly vibrated from side 

 to side, causing the white to 

 flicker 



Ammospermophilus h. harrisi 

 General size greater; body 153.6 mm. 



Tail relatively long, 48 per cent of 

 body. 



Audital bullae slightly higher, more 

 inflated. 



External ear slightly smaller, 6.9 mm. 



General coloration darker, distinctly 

 brownish dorsally. 



White side-stripes narrower and not 

 so long. 



Tail beneath grizzled gray. 



Tail carried vertically or inclined 

 backward or forward, but snow- 

 ing no conspicuous white. 



Tail often waved or jerked in fore- 

 and-aft arc. 



In the matter of audital bullae, liarrisi parallels Thomomys chrys- 

 onotus, in the increased size of bullae on the Arizona side of the river. 

 A curious anomaly appears in the reverse proportion in the size of 

 the external ear in the two squirrels, leucurus of the California side 

 having larger ears than harrisi. There are similar reverse ratios 

 between ears and bullae in certain southern California Perognathus. 



The most striking difference between Ammospermophilus harrisi 

 and A. leucurus is. as noted in the table, in the color of the under 

 surface of the tail and in the manner of carrying this appendage. 

 It may be inferred that the musculature connected with the tail is 

 epiite differently developed in the two species, together, of course, 

 with correlated nervous and circulatory differences. 



