306 Bulletin American Museum of Natural History. [Vol. XXXIV, 
is the only genus south of the Mexican plateau with the premolar formula ¢; 
and the only other genera of Central America, or of North America, with 6 
instead of 8 mamme are Microsciurus, Syntheosciurus (the latter known 
only from the mountains of Chiriqui), and Baiosciurus of Nicaragua and 
eastern Mexico, north to Tamaulipas. Syntheosciurus, with its grooved 
upper incisors, two upper premolars and rather peculiar skull, is not closely 
related to Mesosciurus. The only point of close agreement between Bavo- 
sciurus and Mesosciurus is in the number of mamme, which is insufficient 
to outweigh the other differences between them. Bazosciurus superficially 
recalls Leptosciurus, but in essential characters they are widely dissimilar. 
Finally then we may conclude that the South American genera at present 
resident along the direct highway of migration between North and South 
America were disintegrated from the primitive stock and received their 
present impress long ages ago. 
Guerlinguetus, aside from the difference in the mamme formula, is not so 
very unlike the hof'manni section of Mesosciurus, these two types being quite 
similar in tooth characters and in the general form of the skull. Gwerlingue- 
‘us has developed a relatively much longer tail, and a quite different pelage 
in the typical forms, the latter readily explainable on the basis of the very 
different environment of the two groups. ‘These two genera could have 
originated, at no very remote date, from a common ancestral type. 
The giant squirrels of the Amazonian lowlands may well be supposed, 
on geological and geographical grounds, to be of more recent origin than 
the forms of the older land surfaces of South America; yet the element of 
migration is a possible source of grave uncertainty. From the viewpoint 
of present evidence, the large squirrels of the Amazonian woodlands may 
be said to resemble, superficially at least, the fox squirrels (genus Para- 
sciurus) of middle North America (mainly southern United States and 
Mexican tableland), as in size, proportions, the heavy bushy tail, the 
possession of only one upper premolar and 8 mamme, and still further in 
the elongate, low-crowned skull. On the other hand, they differ quite 
strongly in tooth structure, the teeth being heavy and strong, with well- 
developed cusplets on the outer border of the upper molars in Parasciurus, 
with all these conditions reversed in Uroscvuwrus and its near affines in South 
America. On the other hand, the differences are far greater between 
Urosciurus and the various types of North American and Central American 
squirrels with the premolar formula ? than between Urosciwrus and Para- 
sciurus. We are left then to the supposition of an early extended migra- 
tion of an original stock from North America to Brazil, with subsequent 
essential modification, or the supposition of an evolution from some early 
type or types from which the other existing South American genera of 
squirrels have been derived — with always the exception of Sctwrillus! 
