722 SCIURID^— CITELLUS 



lateral bristle fringes, especially noticeable along the outer 

 margin of each hand. The tail is much shorter than in 

 Sciurus, its length in European species ranging between 

 one-fifth and one-third, instead of being about two-thirds, of 

 the head and body measurement ; cylindrical at its base, it 

 is densely clothed with hair ; and though far less bushy than 

 in the Common Squirrel, it shows some trace of vertical flatten- 

 ing towards its termination. 



The external differences between Citellus and Sciurus seem 

 for the most part to be directly correlated with the more 

 earth-bound habits of the Sousliks. Change of station has 

 brought with it change of food, and this seems to be the 

 chief factor which has influenced the development of the 

 peculiarities of the dentition and skull in Citellus. The 

 incisors are less adapted for gnawing than in Sciurus, being 

 relatively weaker, less compressed laterally, and more nearly 

 cylindrical in cross section. The cheek - teeth are - on 

 each side, as in Sciurus, p 3 being well developed though 

 small. In form they are clearly more specialized than in the 

 squirrel, and are adapted for the treatment of a coarser diet. 

 In each upper cheek-tooth (except /3) the single inner 

 cusp and the two cross crests, which connect the inner cusp 

 with the two chief outer tubercles, have become higher rela- 

 tively and more apparent, forming a conspicuous U-shaped 

 pattern in moderately worn specimens. Corresponding, though 

 less obvious changes have taken place in the lower cheek-teeth. 

 The skull and mandible under the influence of stronger muscles 

 have become relatively robust, and far more massively con- 

 structed than in Sciurus. Earth-bound habits call for less 

 mental activity than does an arboreal mode of living ; the 

 cerebral hemispheres are less developed than in Sciurus, and 

 the fronto-parietal region is therefore flatter and much less 

 boldly convex. 



The advance of civilization, by bringing the waste places 

 under cultivation, has brought the Sousliks into direct conflict 

 with man at many points in their range. Cultivated plants 

 have a higher food value than the natural herbage ; and when 

 man substitutes the former for the latter the Souslik population 

 increases rapidly in numbers. In recent years these animals 



