Alactaga is much like Euchoreutes; it has five toes, a cylindrical tufted tail, the hairs at the end distichous, smooth incisors, and a premolar present in the upper jaw. It also differs from Euchoreutes by the much smaller auditory bulla as well as in the fact that the infra-orbital foramen has no separate passage for the nerve, which passage is to be distinguished in both Dipus and Euchoreutes. The best-known species is the Siberian Jumping Rabbit, A. jaculus. Beneath the ends of the three main toes of the feet are remarkable fan-shaped pads. In A. decumana the body and tail measure 7 and 10 inches respectively, the ears 2 inches. Platycercomys, a fourth genus of the family, is much less known and is to be differentiated from the last three genera by the fact that it has no premolars at all, the grinding tooth formula being thus 3/3. The tail too is flattened and "lancet shaped." It extends from Siberia to Nubia, and thus just enters the Ethiopian region.
The above are the more typical Jerboas. There remain several forms which are not at all Jerboa-like in their way of life, but are nevertheless, on anatomical grounds, placed with them. Zapus, an American genus, with the exception of one Palaearctic species, is transitional in that its hind-legs are rather long, but there is not so much difference between them as in the typical Dipodidae. Sminthus is at the opposite extreme to Dipus. Its feet are short and of equal length; it climbs in trees, and may perhaps be looked upon as nearest of all Dipodidae to the ancestral form of the group.
Fam. 8. Pedetidae.—The genus Pedetes contains but one species, P. caffer, the Cape Jumping Hare. The animal suggests a large Jerboa in appearance on account of its jumping habits, the long hind-limbs, and the long tail. The length of a fair-sized example is some 17 inches, with a tail of the same length. The eyes and ears are large. The hands are five-fingered and the feet only four-toed, the hallux being of course the absent digit. In the skeleton it is interesting to note that the second and third cervical vertebrae are so close together that there can be no free movement; interesting because in Dipus the cervicals are actually ankylosed. The dorsal vertebrae are twelve. The small intestine is long, measuring 7 feet 4 inches, while the caecum is short, being only 8 inches long. The large intestine is 3 feet 10 inches long. The gall-bladder appears to be
