26 NELSON 



Distribution. — All of Central America and Mexico (except extreme 

 northern part). Ranges from Arid and Humid Tropical zones up to 

 Boreal. 



External characters. — Form and color extremely variable : body 

 usually rather slender ; tail long, narrow ; pelage coarse, stiff, almost 

 bristly, in typical species. 



Cranial characters. — Premolars \. Skull short, broad, depressed ; 

 brain case not highly arched but expanded laterally and widening to 

 occiput; interorbital area broad, rostrum short, broad, and heavy; 

 nasals about equal to or shorter than interorbital width. 



General notes. — S. hypopyrrhus Wagler was named as the type 

 of Echinosciurus., but most of the red bellied Central American 

 squirrels were given that name at the time Trouessart wrote, and he 

 no doubt had in mind some bristly haired species from Central Amer- 

 ica instead of VVagler's animal. However, Wagler's hypopyrrhus 

 belongs to the same natural group and must therefore be retained as 

 the type. Most of the large squirrels in tropical North America belong 

 to this group. The skulls are distinguished from those of other large 

 North American species by the combination of two upper premolars 

 with a short broad rostrum and flattened interorbital region. Echino- 

 sciurus contains S. aureogaster., S. poliopus., S. yucatanensis, S. 

 collicei^ S. truei, S. sinaloensis., S. nelsoni., S. socialis., S. griseo- 

 yiavus., S. goldmani., S. managuensis, S. boothice^ S. adolphei and 

 S. thomasi with their subspecies. These species occupy nearly all of 

 the hot coast region of tropical North America and range thence over 

 the mountains of Central America and southern Mexico. Those of 

 the highlands of Guatemala and southern Mexico are distinguishable 

 from those of the hot coasts and the interior of Central America by their 

 much longer and softer pelage. " In the former series are 6^. aureogaster., 

 S. poliopus, S. truei, S. sinaloensis and S. griseojlavus, with their 

 subspecies. The species of the hot districts on the coast and south- 

 ward are remarkable for the stiff, harsh and usually shining long hairs 

 of the back — the character upon which the name of the subgenus 

 must have been based. S. yucatane?zsis with white ear tufts, and S. 

 poliopus with a comparatively long, slender rostrum and soft pelage 

 are least typical. 6". aureogaster hypopyrrhus., the type, is about 

 midway, in the character of its pelage, between the soft haired species 

 of the highlands and the harsh haired species of Central America. 

 The wide geographic range and large number of species of this 

 group are accompanied by a greater amount of specific variation in the 

 skull than is shown in the smaller groups. In the species of Central 



