684 SCIURID^ 



arboreal and terrestrial squirrel-like rodents, but does not 

 include the volant genera, which are placed in a special 

 family — the Petauristidce. 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 CastoHdcB. Thus the orbits are always partly 

 roofed by the considerable supraorbital processes of the frontals. 

 The auditory bullae 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 featnres 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 (/') 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 Euxerus or the Asiatic Eutamias, the brain-case 

 retains a form which recalls that of the least modified Hystrico- 

 morpha. The upper incisors extend backwards into the 

 maxillee, 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 penis; Thomas, who calls this bone the 

 baculum, has recently shown it to be subject to great and 



