48 Zoology of Colorado 



down the valleys, and Mr. Alfred Wheeler, who has obtained a 

 great many skins in the region about Boulder, is unable to see 

 any racial difference between the upland and more lowland 

 specimens. The distribution of the races of beaver is peculiar. 

 The frondator type, dark red brown, the under fur dusky black, 

 the skull broad, extends from northern Sonora up the Rocky 

 Mountains. In the Rio Grande Valley of New Mexico is a 

 different form, C. c. mexicanus of Bailey, of medium size, the 

 colors dull and pale, with very little chestnut at any season. 

 The skull is relatively short, wide and high. The type came 

 from six miles below Ruidoso. In North Dakota is a race mis- 

 souriensis of Bailey, slightly smaller than canadensis, and much 

 paler and duller brown; the skull more triangular in outline, not 

 so massive and heavy. It may still be a question whether our 

 beaver is identical with that described from Sonora, as Taylor 

 states that the under fur of true frondator is cinnamon-drab or 

 light-drab. Living on the beaver, as an external parasite, is a 

 very remarkable beetle, called Platypsyllus castoris of Ritsema. 

 Mr. Ralph Hubbard found specimens on beavers from Marshall 

 and South Boulder Creek. This parasite, constituting a distinct 

 family of Coleoptera, was first found on American beavers in the 

 Zoological Garden at Amsterdam, but it is now known to occur 

 on wild beavers, both in France and America. The European 

 and American beavers have evolved into distinct species, but 

 their more conservative parasite has remained the same. The 

 flattened tail of the beaver, adapted for swimming, is a very 

 peculiar feature. It is interesting to note that the musk-rat, 

 similarly aquatic, has also a flattened tail, but the flattening is in 

 a different (vertical) plane. 



The Muroidea, or mice and rats, constitute a very complex 

 group, the classification of which is by no means easy. Miller 

 and Gidley recognize only a single family, the Cricetidae, native 

 in America. The Muridae have been introduced by man. It 

 might be believed that the whole superfamily reached America 

 in comparatively recent times; but there is an extinct Muroid 

 family, Ischyromyidae, in the North American Oligocene. Species 

 of Ischyromys have been found fossil in Colorado.* Working on 



♦See Troxell, Amer. Journ. Science, Feb. 1922. 



