642 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
strongly developed than in Sciurus, are placed higher up on the skull, and 
unite more anteriorly to form a short interparietal crest ; in Cynomys, these 
ridges meet in very old individuals at only a short distance behind the post- 
orbital processes, and form thence posteriorly a slight interparietal crest. 
Though present in even comparatively young specimens, this crest has a less 
anterior extension. In Arctomys, the interparietal and occipital crests are, in 
the adults, strongly developed. The antero-posterior thickness of the incisors 
varies with the general form of the skull and with the nature of the food. 
In the arboreal Squirrels, which feed largely upon hard nuts, the antero- 
posterior diameter at the base is two to three times greater than the trans- 
verse; in Tamas and the most Sciurine Spermophiles (as Sp. grammurus), the 
antero-posterior diameter is only twice the transverse or less; in the more 
slender Spermophiles, with elongate skulls, which feed on herbaceous vegeta- 
tion, the two diameters are more nearly equal, and the teeth relatively much 
smaller and comparatively slender. ‘The outer edges are also rounded, while 
in the arboreal Squirrels, and those with strong incisors, the outer side is flat. 
GEOGRAPHICAL DISTRIBUTION AND VARIATION. 
Representatives of the Sccuride are found throughout all the continental 
lands of the globe except Australia, but they are by far the most numerous 
in the Northern Hemisphere. Sciwrus, with the exception named, is nearly 
cosmopolitan; Cynomys, on the other hand, is restricted to a comparatively 
small portion of North America. Arctomys, Tamias, Spermophilus, and Sci- 
uropterus are found throughout the temperate and cold-temperate portions of 
the Northern Hemisphere, Spermophilus reaching its greatest numerical devel- 
opment in North America. Pteromys occurs only in the tropical portions of 
Asia, and Xerus is limited to Africa. The Sciuride are represented in South 
America by only the single genus Sciwrus, and by comparatively few species, 
which are generally not numerously represented in individuals. Passing 
northward, the Sciwri are much more abundant in Central America and Mex- 
ico, the genus being represented in Southern Mexico by more species than 
elsewhere, in any equal area, in the New World. In the United States, ° 
rarely more than three species occur anywhere together, while north of the 
United States the genus is represented by only a single species. Europe 
has likewise only a single species, but a considerable number occur in Asia 
and others in Africa. 
