722 SCIURIDAZ—CITELLUS 
lateral bristle fringes, especially noticeable along the outer 
margin of each hand. The tail is much shorter than in 
Scturus, 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 Cr¢ed/us and Sczurus 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 Cr¢e//us. The 
incisors are less adapted for gnawing than in Sczwvus, being 
relatively weaker, less compressed laterally, and more nearly 
cylindrical in cross-section. The cheek -teeth are * on 
each side, as in Scewrus, 63 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 Sczwrus. Earth-bound habits call for less 
mental activity than does an arboreal mode of living; the 
cerebral hemispheres are less developed than in Sczwsus, 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 
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