Vol. XXVI, pp. 35-38 February 8, 1913 



PPxOCEEDINGS 



OF Till 



BIOLOGICAL SOCIETY OF WASHINGTON 



ACHROMATISM IX NEOTOMA MEXICANA FALLAfi 

 FROM COSTILLA COUNTY, COLORADO. 



V 



BY KDWAKD R. WARREN, 





While collecting at San Acacio, Costilla County, Colorado, 

 in June, 1912, I found a well marked case of dichromatism in 

 Neotoma mexicana fallax Merriam. Briefly stated this took the 

 form .if (lark-colored underparts, with the tail dark below as 

 well as a hove, and the feet dark-colored down to the toes. As 

 it is of a somewhat melanistic character 1 will, for convenience, 

 refer to the specimens hereafter as melanistic. 



The animals were captured in a low range of hills ahout three 

 miles west of the town (the northwesterly end of the San Luis 

 Hills), this being the first time the species had been taken in 

 the San Luis Valley, Colorado. As usual, they were found 

 a,mong rocks, in this case an eruptive rock, dark brown in 

 color, with a slight tendency toward a reddish tinge. Pieces 

 of loose rock laying about were often a rusty red, hut I saw no 

 such rock in place. The soil about the rocks was not especially 

 dark. I took careful note of these matters, thinking they might 

 have some hearing on the coloration of the rats. My traps 

 were set on one of the low ridges which form the range of hills, 

 along the outcropping ledges on the summit, strung out for a 

 distance of several hundred yards, and beginning near where 

 the hill began to ascend from the prairie. 1 noticed that none 

 of the melanistic rats were taken beyond a certain point, per- 

 haps two hundred yards from where my trap line began. Tins 

 may have been merely a coincidence, or it may mean that 

 there was a family of the melanistic animals inhabiting that 

 9— Proc. Biol. Soc. Wash., Vol. XXVI. L913. (36) 



