EXTERNAL FEATURES 



13 



while the weight of the Homodontomys was 78 per cent of 

 the same animal. 



It is found that in striking an average of both males and 

 females, the length of tail as taken in the flesh is 93 per cent 

 of the head and body in Homodontomys, 76 per cent in 

 Neotoma, and 77.5 per cent in Teonoma. 



Although there are many exceptions, probably caused by 

 some inhibiting factor or compensating character in the com- 





Fig. 1. Palmar and plantar aspects of (A) fore and (B) hind feet of 

 Teonoma, showing plan of pads, and carpal vibrissal area. 



position of certain mammals, it may be stated as a rule that 

 arboreal specialization in a mammal accompanies either 

 lengthening of the tail or an increase in the length of hair 

 upon the sides of that member {e.g., many s(iiiirrels). This 

 is an arboreal speciaHzation, but variation in this respect is 

 found in mammals that are not truly arboreal. Thus, 

 within certain groups of rodents, those (other than saltatorial 

 forms) which dwell amid a plains environment have the 

 shortest tails (Lagiinis, Citellus beldingi, certain hamsters, 



