OF THE BEAVER AND OF SOME SQUIRRELS. 1183 



b' . Cavity of the ear, when folded, mainly closed by antitragus, 



the inferior continuation of the overfolded anterior border 



not forming a hairy thickening within the cavity. 



c. Antitragal thickening set liigh up near the middle of the 



posterior wall of the cavity ; pinna reduced. 



d. Margin of ear a thickened rim not standing away from the 



head Geosciurus. 



d'. Margin of ear standing away from head dorsally . . . Xerus, Euxerus, 



Atlantoxerus. 

 c'. Antitragal thickening occupying normal position and form- 

 ing the postero-inferior wall of the cavity ; pinna standing 



well away from head dorsally Tj'pical Squirrels, Tamias, 



Petam'istidse. 



Although Castor is isolated at the head of this table, it can 

 hardly be claimed that its ears differ more from those of typical 

 Squii-rels than do the ears of Geoscmrus. Section c' is roughly 

 divisible into two groups, exemplified respectively by Sciurus and 

 its near allies, Tamias and the Petauristidfe, which have long 

 or comparatively long ears, and by the tropical African and 

 Asiatic genera Fanisciurus, Heliosciurus, CallosciurihS, and 

 Ratufa, where the ears are shorter and less tubular at the base. 

 But these groups seem to intergrade. What the ears of Rhei- 

 throsciurus are like apart from length has yet to be ascertained. 



The Feet. 



The feet of the typical arboreal Squirrels of the family 

 Sciuridae have many chai^acters in common. In the fore foot 

 the first digit is reduced to an apparently functionless excrescence 

 springing from or imbedded in the side of the inner carpal pad, 

 and carrying a nail. Of the remaining digits, the fourth is the 

 longest, the diiTerence between the fourth and the fifth or second 

 being considerable. The fourth and the third are more closely 

 united at the base than they are to the fifth or second respec- 

 tively, and the middle line of the artiodactyle foot passes between 

 them. The digital pads and claws are Avell developed. The 

 plantar pad is always represented by three well-defined elements. 

 The undersides of the digits and of the sole are typically naked 

 — hairy in some northern forms in winter, — and there are two 

 well- developed carpal jDads, of which the inner is the larger of 

 the two. The tuft of carpal vibrissre is always present and 

 large. 



The digits of the hind foot resemble those of the fore foot 

 tolerably closely in structure and relative lengths, the fourth 

 being the longest ; but the first digit is well developed and 

 resembles the others, except that it is much the shortest of the 

 series. The well-developed plantar pad consists of four lobes. 

 The metatarsal area is for the most part naked, the heel alone 

 generally being hairy, and is supplied with two pads, an inner, 

 which is the larger, and an outer, which, in addition to being 

 smaller, is set more forwards. Sometimes one or both of these 

 pads is suppressed, in the case, for instance, of Squirrels, which 

 have hairy feet in the winter, e. g. S. vulgaris. 



