660 MR. R. I. POCOCK ON THE EXTERNAIi ; 



That Zcipus belongs to the same gi'oup of Rodents as Jacidits 

 has long been admitted, and is disputed, so far as I am aware, by 

 no one acquainted with this order. The evidence rests mainly 

 upon certain cranial characters connected more particularly with 

 the zygomatic arch and the preorbital foramen, the structure 

 of which distinguishes that genus from the Muridee, despite the 

 generally murine shape of the skull. It is also conceded that 

 Slcista, formeidy knovvn as Sminthus, is tolerably closely related 

 to Zapus. With regard to Pedetes, my pretext for including 

 that genus in this paper is its former ascription to the same 

 section * as Zapus and Jaculus, although now recognised as 

 representing a quite distinct family, sometimes associated with 

 the Hystricomorpha, together with the family Anomaluridpe. 



External Appearance. 



The general appearance of Sicista and Zapus may be described 

 as murine. They have a long mouse-shaped head and compara- 

 tively small diurnal eyes, and, although the hind legs of Zapus 

 are elongated and saltatorial, they are not very much longer rela- 

 tively than in our common wood-mouse, Apodemtos sylvaticus. The 

 Jerboas, on the other hand, are not the least like typical murine 

 rodents in appearance. The head is very large, wide, and, as a 

 rule, short especially in the snout, which is deep from above 

 downwards, and the eyes are relatively enormous and nocturnal. 

 The hind legs are of great length and the fore limbs are short. 

 A Jerboa with the fore limb, measured from the elbow, of 

 approximately the same length as that of a Zapus has the foot, 

 measured from the heel, about twice as long as that of Zapus. 

 Hence, when progressing on all fours, the gait of a Jerboa is 

 exceedingly awkward, owing to the great elevation of the hind 

 quarters. They seldom adopt that mode of progression, usually 

 employing their hind legs alone. When moving at speed they 

 take prodigious hops, like a Kangaroo; but they frequently go at 

 a more leisurely pace, trotting or striding along like an ostrich 

 or a human being, an action of which Kangaroos, and, pre- 

 sumably, Zapus also, are quite incapable. 



The great variation in the size of the ears affects the appearance 

 of different species of Jerboas when compared with one another. 

 They also differ in the shape of the muzzle. As above stated, the 

 muzzle is typically short and truncated ; but in Euchoreutes, 

 hence named naso, it is relatively long. Presumably also 

 Allaclaga, although I have never seen a living example, must 

 have a differently shaped muzzle from Jacidtcs on account of the 

 pi^ominence of the upper incisor teeth, which form an angle 

 instead of, as in Jaculus, a continuous curve with the nose. 

 Scarturus is intermediate between the two genera just mentioned 

 with respect to the direction of these teeth. 



* See Flower and Lydekker's ' Mammals, Iiiving and Extinct,' p. 480 (1891^, 



