642 MONOGKArnS OF NOIITII 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 arc, 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 Tamias 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 Hat. 



GEOGRAPHICAL DISTRIBUTION AND VARIATION. 



Representatives of the Sciurida: are found throughout all the continental 

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

 in the Northern Hemisphere. Sciurus, 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- 

 vropterus are found throughout the temperate and cold-temperate portions of 

 the Northern Hemisphere, Spennop/tilus 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 Sciuridce are represented in South 

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

 which are generally not numerously represented in individuals. 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. 



