622 MONOGRAPHS OK NORTH AMERICAN RODENTIA. 



ears, though very small, provided with a distinct auricle. Fore claws mod- 

 erately developed. Containing the smaller species of the family. Distributed 

 from British America to Mexico, from the valley of the Mississippi to the 

 Pacific. 



Readily distinguished from Geomys by the characters given in the fore- 

 going paragraph, the expressions used being antithetical to those of the diag- 

 nosis of Geomys on p. 611. 



The numerous species of this genus described by Richardson, LeConte, 

 Baud, and others are reducible to one, with three geographical races. The 

 following schedule exhibits the relations of the several forms as treated by 

 the two latest systematic writers on this genus: — 



Baibd, 1857. Coues, 1875. 



1. Thomomys bulbivorus .. ~\ ( ~\ & ® £? » 



2. Thomomys latiaps > Pacific coast region < bulbivorus . . 



3. Thomomys douylassii. .. j t. 



4. Thomomys t borealis . . . ~\ ( 

 ... ) Northern Interior < i 



C. Thomomys " lalpoides". J (. 



7. Thomomys umbriuus . ..' 



- 



J£ .a 



<h ^ ^ 



x. so S a 



% | ° £ 



5. Thomomys rufesctns ... ^Northern Interior ■( taxpoides I t ^ o .2 



^ __, n £ 



£ "3 33 



* « « g 



— % 2 



<2 P. '« ,5) 



> Southern Interior and Lower California •; t 



'J I. 



^ .-, ,. , /ouuuiciu lurau IIJJU una vutuwiu s UMBRINUS . 



8. Thomomys fiilvus . 



For a full discussion of the subject, reference may be made to the orig- 

 inal memoir. The following is an epitome of the results attained in the dis- 

 crimination of the three forms: — 



Size. — The Northern Interior form and the Pacific Coast form are of 

 the same size ; the Southern Interior form averages an inch or two less in 

 total length than the other ; but large specimens of the latter, and small 

 examples of the two former, overlap each other in stature. 



Form — The Northern Interior race and the Northern styles of the Pacific 

 Coast race have larger fore claws than the Southern style of the Pacific Coast 

 race or than the Southern Interior race. The difference is sufficient to make 

 the whole hand of the former about equal to the foot, while, in the latter, the 

 hand is usually shorter than the foot. But this is only true as a rule; there 

 are many exceptions. In all three of the forms, the tail, measured from its 

 true base, ranges from one-third to one-half the total length of head and body, 

 though only rarely reaching either of these extremes. Nothing can be pred- 

 icated on this score. 



Color. — The Northern Interior race is a rat-colored animal, hoary-gray 

 underneath, with white tail and feet, much white about the mouth, and no 



