526 



PROCEEDINGS OF THE NATIONAL MUSEUM 



Mk 



b c d 



Figure S. — Comparative length of hair of terminal portion of tail of: a, arboreal peromy- 

 scine, Nyctomys; b, arboreal peromyscine, Rhipidomys; c, arboreal oryzomyine, Oryzomys 

 bicolor; d, terrestrial oryzomyine, Oryzomys laticeps. 



pilosity of the tail (figs. 5, 6) varies considerably but is never as great 

 as in such related arboreal cricetines as Rhipidomys and Nyctomys. 

 The thin pencil and elongated hairs of the terminal one-fourth or 

 one-third of the tail are probably sensory and used by the mouse for 

 detecting movements in the rear. 



Discussion: Hind foot length of arboreal mice of the subgenus 

 Oecomys averages less than 21.5 percent of combined head and body 

 length. The hind foot of predominantly terrestrial oryzomyines aver- 

 ages more than 21 percent. The arboreal or terrestrial pes could, there- 

 fore, have evolved from an ancestral type with average length from 21 

 to 21.5 percent of combined head and body length. Specialization of 

 the hind foot of the evolving arboreal cricetine is expressed first by a 

 lateral broadening of the metatarsus. As the animal becomes increas- 

 ingly dependent on its arboreal habitat, the metatarsus becomes 

 more flexible, the first and fifth digits more powerful and opposable, 

 and the plantar tubercles modified for grasping. In contrast, the hind 

 foot of the highly specialized cursorial or saltatorial terrestrial crice- 

 tine becomes narrower and longer relative to head and body length, 

 the metatarsus rigid, and the first and fifth hind digits reduced. 



A tail length approximately equal to head and body combined 

 appears to be the generalized condition in cricetines. In the evolu- 

 tion of ambulatory, terrestrial species, notably volelike and fossorial 

 forms, the tail becomes shorter. In arboreal, saltatorial and some 



