1915.] 



IKTEODUCTION. 



n 



lie, T. mewa from Fresno Flat, Cal., and T. pascalis from Fresno, CaL, 

 show large white patches on backs, arms, and breasts, and one of T. 

 yelmensis from Tenino, Wash., shows white patches on one side. The 

 white patches below, on chin, breast, or belly, are often constant and 

 furnish good specific or subspecific characters. Two specimens of 

 T. aureus from Shiprock, N. Mex., are light gray all over from a pre- 

 dominance of white or light gray hairs. One of T. nevadensis from 

 Battle Mountain, Nev., is hoary over the head and back from a gen- 

 erous sprinkling of white hairs. A specimen of T. fossor from the 

 Cochetopa Forest, Colo., is also gray over the head and back in the 

 same manner, but this may belong to another series of abnormal color 

 variation, caused by ticks or skin disease (fig. 4), 



B2045-122 



Fig. 4.— Skins of Thomomys (U. S. Nat. Mus. specimens), showing abnormal pelage conditions. 



1. Thomomys fossor, No. 137661, female adult, Arapahoe Pass, Colo., July 5, 1905. The abnormal white 



specking is probably due to tick bites. 



2. Thomomys fuscus fuscus, No. 72949, female adult. Tobacco Plains, Mont., July 8, 1895. The white 



on head may be due to ticks, mange, or skin disease. 



MELANISM. 



Melanism is much more common than albinism, as one species, T. 

 niger, is known only from pure black individuals, and six other spe- 

 cies are more or less melanistic. In T. townsendi, from southern 

 Idaho, there are 5 black specimens and 12 of the normal gray. The 

 Biological Survey series contains 36 normal yellow-gray, 18 black, 

 and 10 dark gray individuals. In T. orizahx, of southern Mexico, 

 14 out of 16 specimens from the type locality on Mount Orizaba are 

 black. Of 8 specimens of- T . umbrinus from Boca del Monte 4 are 

 almost black and 4 are dark brown. In a series of 11 specimens of T. 

 atrovarius, 3 are almost black. One specimen of T. peregrinus is fully 

 melanistic, but dichromatism may be rare in the species, as speci- 

 mens from many other localities are normal. In a large series of 

 specimens of T. alticolus from southern Lower California 4 are so dark 



