684 SCIURIDZ 
arboreal and terrestrial squirrel-like rodents, but does not 
include the volant genera, which are placed in a special 
family—the Petauristiede. For our knowledge of the status 
of the group and the classification of its members we are 
chiefly indebted to the work of Winge, Forsyth Major, and 
Thomas. 
In a few respects the family stands on a somewhat higher 
plane than the Castoride. Thus the orbits are always partly 
roofed by the considerable supraorbital processes of the frontals. 
The auditory bulle are divided internally by bony septa. The 
thumbs are in all reduced to short stumps. But apart from 
these characters and from the remarkable degree of specialisa- 
tion evinced by the masseter muscles and the skeletal parts 
under their influence—a specialisation which, as shown above, 
is common to all Sciuromorpha—the members of this family 
retain many primitive features which stamp them as, in these 
respects, the least progressive of the Simplicidentata. 
The dentition includes typically two functional premolars 
above and one below on each side, and these are preceded by 
well-developed and for a time functional milk molars; but in 
many forms the anterior upper premolar (f°) is reduced or 
absent. The cheek-teeth, although showing from genus to 
genus a wide range of variation in structure, particularly in 
the degree of their progress towards lophodonty or hypsodonty, 
are always of a brachyodont type and are implanted solely by 
their distinct roots. In the skull the jugals are always large, 
articulating in front with the lachrymals; the bodies of the 
maxillaries are always shallow ; and where least modified, as in 
the African Auxerus or the Asiatic Hutamtas, the brain-case 
retains a form which recalls that of the least modified ystrzco- 
morpha. The upper incisors extend backwards into the 
maxilla, but terminate distinctly in advance of the premolars ; 
the lower incisors terminate in the ascending rami of the 
mandible, but little above the molar level. 
In the skeleton there are twelve or thirteen pairs of ribs; 
a well-developed clavicle; and the fibula is distinct from the 
tibia and does not articulate with the calcaneum. There is a 
well-developed os penzs; Thomas, who calls this bone the 
baculum, has recently shown it to be subject to great and 
