32 



NORTH AMEEICAN FAUNA. 



[no. 39. 



Genus THOMOMYS Wied. 



Thomomys Wied, Nova Acta Acad. Gees. Leop.-Carol., XIX, pt. 1, 378, 1839. Type, 

 Thomomys rufescens Wied. 



GENERIC CHARACTERS. 



Upper and lower molars with both anterior and posterior enamel 

 plate always present. Upper incisors with plain, slightly romided ante- 

 rior surface, a minute groove near inner edge of each. In some species, 

 notably T. hulhivorus, T. leucodon, and T. umbrinus, the groove is very 

 obscure and can be detected only with a lens ; in others, especially the 

 monticola group, it is relatively deep, well defined, and readily seen 

 with the naked eye. In species where the inner groove is obscure there 

 is often a trace of a median line of depression along the front of the 

 incisor, but it is a mere trace, difficrdt to detect and not sufficiently 

 constant to serve as a diagnostic character. Fore feet relatively much 

 slenderer and claws lighter than in other genera of the family, but 

 claws long and weU adapted to burrowing. 



GROUP RELATIONSHIPS. 



In the present paper the species are arranged so far as possible in 

 accordance with their relationships. There are no superspecific divi- 

 sions of sufficient value to be accorded subgeneric rank, but related 

 species have characters in common which distinguish them as a group 

 from other groups. 



The lower outline of the rostrum is the strongest division character^ 

 and this, though slight, seems clearly to separate two great groups. 

 In one group the rostrum is deep and heavy in side view, and its 

 lower outline slopes rather evenly from the anterior base of the upper 

 molars to near the base of the incisors. This group includes T. hulh- 

 ivorus, T. townsendi, T. hottse, T. alpinus, T. perpallidus, T.fulvus, 

 T. umbrinus, and related species. In the other group the rostrum is 

 relatively slender in side view and its lower outline is abruptly arched 

 in front of the upper molars. This group includes T. talpoides, T. 

 fossor, T. douglasi, T. monticola, T. fuscus, and related species. In 

 the text this division falls on page 96, the slender-rostrum group 

 beginning with T. talpoides and continuing to the end. The species of 

 these two groups seem not to intergrade. 



HEAVY ROSTRUM GROUPS. 



In the heavy rostrum groups are seven divisions, not all of equal 

 distinctness. 



Thomomys hulhivorus stands by itself because of its concave ptery- 

 goids, small claws, and peculiar fur; but it is not very distant from 

 the hottx group with which it agrees in arrangement of inammse in 4 

 pairs. 



