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MOKOOKArnS OF NORTH AMERICAN RODENTIA. 



strongly dcvelopcii tlinn in Sciurui, aro placeil higher up on the skull, and 

 unite more antcriurly to form a short interparietal crest ; in Cynomyt, 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. 

 Thougii present in even comparatively young specimens, this crest has a less 

 niitcrior extension. In Arctomyg, the interparietal and occipital crests are, in 

 the adults, strongly developed. The an tcro- 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 Tamias and the most Sciurinc Spermophiles (as 8p. grammurut), the 

 antcro-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 



OEOORAPniCAL DISTBIBDTION AND VARIATION. 



Representatives of the Sdvridee are found throughout all the continental 

 lands of the globe except Australia, but they are by far the most numerous 

 in the Northern Hemisphere. Sciurug, with the exception named, is nearly 

 cosmopolitan ; Cynomys, on the other hand, is restricted to a comparatively 

 small portion of North America. Arctomys, Tamias, Spermophilut, and Sci' 

 vropterus arc 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 Xerua is limited to Africa. The Sdurida are represented in South 

 America by only the single genus Sciurus, and by comparatively few species, 

 whicli are generally not numerously represented in individuala Passing 

 northward, the Sciuri 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. 



