800 MONOGRAPHS OF NOItTII AMERICAN RODENTIA. 



4.00 to 4.20 in the typical pallidas (from tlic Yellowstone Plains) to 5.25 in 

 the townsendi type of the northwest coast; thai the latter fully equals, and 

 probably exceeds, in size the Siberian type; ami that those from the Pacific 

 coast are considerably larger than those from the interior, from localities hav- 

 ing the same latitude; also, that in the interior there is a well-marked 

 decline in size southward.* The difference in dimensions between the 

 smallest and largest form is about one-fourth of the average size of the whole 

 series. The skull in var. townsendi has an average length of about 1.55, 

 against 1.25 for the same measurement in var. pallidas. 



Specimens from the eastern base of the Rocky Mountains, in latitude 

 49°, and from the Uintah Mountains and the region north of Fort Bridger, 

 present a peculiar dull brownish phase of coloration, with the stripes very 

 distinct and the sides pale. These are about equally well referable to either 

 of three varieties, namely, borealis, pallidas, and quadrivittatus, though per- 

 haps they most resemble borealis. They also show an approach to the dull, 

 heavy tints of var. townsendi; particularly is this true of those from the 

 Uintah Mountains. 



The striking amount of variation with locality among the American 

 forms of Tamias asiaticus, and its exact correlation with differences in the 

 conditions of environment, make this one of the most instructive and interest- 

 ing groups among North American Mammals. These correlations seem to 

 have been first noticed by Dr. J. G. Cooper,! who, as early as 1869, in 

 speaking of the pale form of Tamias "quadrivittatus" of the Upper Missouri 

 Bad Lands, called attention to the difference in color seen between specimens 

 inhabiting the open plains and the forests, and expressed his belief that the 

 pale form of the Plains owed its paler tints and smaller size "to the influence 

 of more sunlight and heat, combined with inferior food''. He adds: — " Varia- 

 tions in color connected with exposure to the sun and heat, are noticed in 

 T. townsendi and T. striatus, as well as in other animals, so that allowance 

 must be made for such influences in the determination of species. The 

 variety found by me in 1863, at the Clickatat Pass, Cascade Mountains, 4,500 

 feet above the sea, and at first named T. cooperi by Professor Baird, is so 

 nearly intermediate between the form found on the west {T. townsendi) and 



* Unfortunately, I inadvertently omitted to take measurements of the northern specimens at the 

 time (three years since) 1 had the whole material before me, which has since been widely dispersed. 

 t Amor. Nat. vol. ii, pp. 5:50, 531. 



