CHARACTERS OF SCARTURUS AND OTHER JERBOAS. 667 



the outer wall of the tubular base there arises a compressed 

 process which projects upwards as a free outgrowth some little 

 distance above the margin of the basal tubular portion. When 

 the ear is expanded this process, which possibly represents the 

 tragus, inclines forwards along the edge of the ear in front of it. 

 The suprat)'agus is a well-defined ridge running upwards and 

 backwards, but stopping abruptly in the middle of the ear far 

 away from its posterior edge ; and the wall of the etir between 

 the posterior end of the supratragus above and the auditory 

 orifice below is folded and creased. (Text-fig. 31, F-H.) 



The reason for the termination of the supratragus in the middle 

 of the ear is this. When the ear is folded, it doubles up exactly 

 lengthwise along the middle line, so that the anterior and posterioi' 

 margins are evenly in contact throughout their length, except 

 close to the basal tubular portion ; but the space that is left 

 between them at this spot is filled by the process above described, 

 and that appears to be its function. The grooving of the wall 

 of the lower portion of the ear is also subservient to this peculiar 

 method of folding, which could not be efl^ected if the rigid supra- 

 tragus extended across the cavity of the pinna. 



By the structure of the ear the genera discussed fall into the 

 following three headings : — 



a. Ear long, folding exactlj' along the middle line, so that its anterior 

 and posterior edges coincide ; a columnar process rising from its 

 basal tubular portion ; supratragus a well-defined thickened ridge. 

 No inturned Hap at the base of the anterior edge in front Fedetes. 



a'. Ear long or coniparativel.y short, not folding so that the two edges 

 of the pinna exactly coincide ; no columnar process rising from its 

 basal portion ; a distinct inturned flap at the base of the anterior 

 edge in front. Supratragus comparatively wealc. 

 b. A small tragus and large antitragus, base of pinna not tubular. 



Zapus, Sicista. 



V . No tragus or autitragus ; base of ear more or less tubular. 



Jaculus, Scirtopoda, Scarturus, ALlactaga. 



The Fore Foot 



In Sicista subtilis, judging from Tullberg's figure, the fore foot 

 is comparatively broad, short, and unmodified ; the four main 

 digits are subequal in length, the third and fourth projecting a 

 little beyond the second and fifth, the latter being a little shorter 

 than the second ; the first, or pollex, is quite short, and springs 

 from the middle of the side of the foot close to the distal edge 

 of the carpal pad. The digital pads are moderate in size and 

 elliptical, and the claws are small. The plantar pad consists of 

 three moderately large interdigital elements, subtriangular in 

 shape and in contact or nearly so. The carpal pads are large, 

 covering about half the area of the sole, the internal element 

 being about twice the area of the external, which is in contact 

 with it. (Text-fig. 32, B.) 



The fore foot of Zapus is comparatively long and narrow. The 

 carpal pads and pollex are tolerably similar to those of Sicista, 



