20 



NORTH AMEKICAK FAUNA. 



[NO. 39. 



Climatic conditions control the time and nature of molt, but not 

 its general plan. Species from high in the mountains and from well 

 north wear heavy winter coats for a longer time: T. talpoides from 

 October to April; T.fossor and T.fuscus from October to May; while 

 those from low hot valleys — T. perpaUidus, T. alhatus, and T. chryso- 

 notus — wear thinner winter coats from about the first of December 

 to the first of March. Species from the California coast show the 

 usual molts, but with slight contrast between summer and winter, 

 and with considerable individual variation in dates of change. The 

 one fully tropical species, T. atrovarius, from western Mexico, is repre- 

 sented by specimens taken from April 5 to August 3, in most of which 

 one molt line appears across the back, and none shows more than one, 

 while there are several without any trace of molt. This condition is 

 rarely found in any other species in summer pelage and is probably 

 due to the tropical climate. Specimens showing a molt line seem to 

 be changing from one thin, harsh pelage to another exactly like it, 

 and whether there is any change to a winter pelage in this species 

 remains to be shown by winter specimens. 



When the young, which are evidently born naked, are old enough 

 to begin moving about, they have a fine, short coat of silky hair, 

 which a little later becomes longer and more lax as well as finer and 

 softer than in adults, and when they are about half grown the molt 

 begins on the nose and progresses slowly as in adults, with but one 

 molt to adult summer pelage. This change is not completed until 

 the animal is almost grown, and while it resembles the faded and 

 old worn pelage of adults, it can be recognized by its finer, softer 

 texture. 



The number and sequence of molt waves can be satisfactorily deter- 

 mined only by a study of living animals as a supplement to the large 

 series of specimens available. 



ABNORMAL PELAGE CONDITIONS. 



ALBINISM. 



Albinism is rare in the genus Thomomys, but a number of spec- 

 imens examined are fully albinistic and others partially so. One 

 albino of T. hottse from San Francisco is in the U. S. National Museum 

 collection, and Townsend speaks of a ^'perfect albino" presented 

 to him at San Diego. ^ Two specimens of T. fuscus in the Biologi- 

 cal Survey collection from Shuswap, British Columbia, are pure white 

 all over. A pure white specimen lacking all of the skull except the 

 front teeth was given me at Scottsburg, Oreg., but the species was not 

 identifiable ; it was probably T. niger, a species otherwise known only 

 from pure black specimens. Specimens of T. awahnee from the Yosem- 



1 Proc. U. S. Nat. Mus. X, 177, 1887. 



