NO. 5 MAMMALS OF PANAMA GOLDMAN I43 



upper and under parts. The tail is edged with rusty reddish instead 

 of grayish white as in the latter form. 



These tiny tree squirrels are apparently not very numerous, or, 

 owing to the density of the forest cover they inhabit, individuals 

 easily escape observation. In allusion to rapid movements the animal 

 has received the native name, in the Canal Zone, of ardita voladora. 

 One of the specimens taken at Gatun was seen running rapidly down 

 the trunk of a tree. I noticed that the tail seemed to extend behind 

 rather stiffly in a straight line with the body. On the top of the hill 

 near the west end of the Gatun Dam one, which had evidently become 

 alarmed at my approach, was seen moving down the trunk of a small 

 tree. When within four feet of the ground it slipped suddenly out 

 of sight on the opposite side before I could shoot. I supposed it had 

 jumped to the ground but found on searching that it had climbed the 

 tree again and was watching me from a perch among some leaves in 

 the extreme top, sitting motionless with its tail curved over the back 

 in characteristic squirrel fashion. 



From the Canal Zone M. a. venustulus ranges eastward to near the 

 Colombian frontier. A specimen collected at Porto Bello was cling- 

 ing head downward, about 20 feet frcta the ground, on the trunk of 

 a large tree giving short squeaking sounds suggesting those of some 

 North American chipmunks. A single example was obtained at 

 2.000 feet altitude on the mountains near Cana. The same mountain 

 slope at 3,500 feet is inhabited by Microsciurus isthmius vivatus 

 which here typifies another species. Anthony (1916, p. 366), who 

 obtained specimens on Mount Tacarcuna, also noted their occurrence 

 in the same general locality and apparently overlapping the range of 

 M. i. vivatus, but he states that venustulus was taken at slightly 

 higher elevations and on the crest and eastern slope of the mountains. 

 His specimens agree closely in color with the type. M. i. venustulus, 

 contrasted with M. i. vivatus has darker, much more finely grizzled 

 upperparts. Specific distinction is, however, better shown in cranial 

 details ; in the skull of M. a. venustulus the interpterygoid fossa and 

 basioccipital are narrower, the maxillae are less extended at the 

 expense of the frontals between the lachrymals and the premaxillae, 

 and the interparietal is rectangular instead of subtriangular in out- 

 line. The type of M. a. venustulus, an adult female, lacks the small 

 upper premolars usually present in Microsciurus. 



Specimens examined: Gatun (type locality), 2; Cana, 1; Mount 

 Tacarcuna, 3 1 ; Porto Bello, 1. 



1 Collection Amer. Mus. Nat. Hist. 

 10 



