CHARACTERS OF SCARTURUS AND OTHER JERBOAS. 671 



farther from the third than tlie latter is from the fourth, these 

 two being closely united proximally. The hallux is moderately 

 long and slender, and rises a little behind the origin of the fifth ; 

 it reaches to the base of the second, which is shorter than the 

 third approximately by its digital pad ; the fifth i-eaching only 

 to the distal end of the narrow webbing tying the third and 

 fourth closely together proximally. The claws are long and 

 slender, and the digital pads elliptical. The plantar pad is repre- 

 sented by four small, isolated, subcircular, interdigital elements 

 set on the granular sole in accordance with the disposition of 

 the digits. The metatarsal pads are represented merely by the 

 internal element, which is a small subcircular pad set one-third 

 of the distance along the metatarsal area between the hallux 

 and the heel. (Text-fig. 32, F.) 



The hind feet of the Jaculidse diflier profoundly from those of 

 Zapus in the disposition of the digits and other particulars. 

 They are markedly perissodactyle, the third digit being the 

 longest, set in the middle line, and symmetrically flanked on each 

 side by the second and fourth, which are equal in length and 

 rise at the same level. Their digital pads are moderately or very 

 large, deep, compressed, and laterally grooved, with lobulate 

 distal edge. The plantar pad is represented by a larger or smaller 

 soft median papilla,' pointed apically. The metatarsus is much 

 longer than in Zapits, and, as is well known, the three bones 

 corresponding to the three main digits are completely fused into 

 one. The first and fifth digits may be absent or present. When 

 present they rise a long way back on the metatarsus, and, since 

 they do not normally touch the ground, have been described as 

 functionless. They are slender, with a small normal digital pad 

 and thin longish claw. The metatarsal bone proper to each 

 i-emains distinct from the median " cannon " bone, and, when 

 both these digits are retained, they spring from the side of the 

 metatai^sus at the same level. Progressive reduction in the 

 number of digits from five to three is shown in the family. In 

 Pifgereimus, AUactaga, and Euchoreiotes the first and fifth are 

 retained. In Scarturu-s the first is lost and the fifth retained. In 

 Jaculus both are lost. The underside of the digits varies greatly 

 in hairiness. In the two Egyptian (i\)Qc\its, Jaculus jacidus ^.n^L 

 Scirtopoda oriPAitalis, they are thickly fringed on each side with 

 long hairs, whicli curve inwards over the undei- surface of the 

 digits a.nd form a soft hairy cushion. Close to the pads these 

 hairs are exceptionally long. They quite conceal the median pad 

 from the lateral aspect, but the pads of the second and fourth 

 digits are not entirely covered by them. In these two hairy- 

 footed species the median plantai' papilla is veiy small and soft, 

 and the digital pads are comparatively small. In Scarturtis, 

 which occurs in the same localities as Scirtopoda orientaUs, on 

 the contrary, the under sides of the digits have no long fringes, 

 and the digital pads are very large and deep, and more strongly 

 grooved and marginally lobate than in Scirtopoda oi'ientalis and 



