SQUIRREL TRIBE. 
57 
The order is divided into those with only one pair of incisor 
teeth in the upper jaw (Simplicidentata) and those with two pairs 
(Dnplicidentata). The first of these suborders contains by far 
the greatest number of species, and is itself divisible into three 
sections, of which the Squirrel, Rat, and Porcupine are severally 
typical. 
The members of the Sciuromorpha, or Squirrel section, are 
distinguished by having at least one pair of premolars, by a 
flattened, not twisted, lower jaw, small perforations on the 
palate, and by the two shin-bones, the tibia and fibula, always 
remaining separate. They generally have well-marked processes 
behind the sockets of the eyes in the skull. To this grouj) 
belong the Squirrels (^Sciuriche), Beavers (^Castorid(r\ &c. 
Although it is doubtful whether they belong to the Sciuro- 
inorpha, here may be mentioned the Scaly-tailed Squirrels 
(^Anomaluridce') of Equatorial Africa (case 12), typically with [Case 12.] 
membranous parachutes like the Flying-Lemurs described 
above (p. 34), and with a series of pointed scales under the 
base of the tail, so placed as to be of use when the animal 
is resting on a vertical tree-trunk, the points of the scales 
sticking into the surface of the bark. Anoynalurn s ( 753 ) and 
Idiurus are Hying forms, but in Zenker ella the parachute 
is absent. Near by are the American Sewellels {Aplo- 
dontiidee), ibrmerly regarded as near relatives of the Squirrels, 
and represented only by the single genus Aplodontia, or 
Jfaplodon ( 754 ). 
The Flying-Squirrels of Southern Asia, Petaurista (725 k [Case 12.] 
726 ), some of the most brightly coloured of all Mammals, 
belong to the family Schirida\ With the exception of the 
flying membrane, there is little structural difference between 
them and ordinary Squirrels. 
The Chipmunks, or Striped Gophers, Tamias (719 to 723 ), the 
Susliks, Sperniopliilus or Citillus (732 to 739 ), and the Marmots, 
Aretomys ov Marmotta (742 to 748 ), live in burrows of their own 
construction. The Common Marmot, Aretomys marmotta (746 j, 
inhabits the Alpine regions of Europe. The North American 
Prairie-Marmots, Cynomys ( 740 ), better known as Prairie-Dogs, 
excavate a large number of deep burrows close together, forming 
