800 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
4.00 to 4.20 in the typical pallidus (from the Yellowstone Plains) to 5.25 in 
the ¢ownsendi type of the northwest coast; that the latter fully equals, and 
probably exceeds, in size the Siberian type; and 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. ¢ownsendi has an average length of about 1.55, 
against 1.25 for the same measurement in var. padlidus. 
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, pallidus, 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,t who, as early as 1869, in 
speaking of the pale form of Tamas “‘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, Cuscade Mountains, 4,500 
feet above the sea, and at first named T. coopert by Professor Baird, is so 
nearly intermediate between the form found on the west (7. townsendi) and 
* Unfortunately, I inadvertently omitted to take measurements of the northern specimens at the 
time (three years since) I had the whole material before me, which bas since been widely dispersed. 
t Amer. Nat. vol. ii, pp. 530, 531. 
