The YELLOW POCKET MOUSE and the LEAST POCKET MOUSE (_Cricetodipus flavus_ and _parvus_) are very minute creatures, only about two inches long in the head and body. The tail is longer than the head and body in the latter, shorter in the former species, and the colour of the fur in both is a pale buff. These species are found in the Rocky Mountains and the region west of that range to the Pacific, the latter being inhabited by the second of the above species. Several species of the genus _Heteromys_ inhabit Central America, and one is found in the island of Trinidad. Nothing appears to be known of their habits. From these we pass as by a natural transition to FAMILY X.--DIPODIDÆ (THE JERBOAS). The JERBOAS are a more extensive and much more widely distributed family of hopping Rodents. In these we find the organisation for jumping brought to greater perfection than in any other group. The body is light and slender, the hind limbs much elongated, the fore limbs very small, and the tail long and usually tufted at the end. The number of toes on the hind feet varies from three to five, and the metatarsal bones are very often united so as to form what is called a “cannon bone” in the Horse. The incisor teeth are compressed; the molars sometimes four, but usually three in each series, rooted or rootless, not tuberculate; the infra-orbital opening is rounded and very large, and the zygomatic arch slender. The great home of these animals is the vast steppe region which stretches from South-eastern Europe across the greater part of Central Asia, but they extend southwards round the eastern extremity of the Mediterranean, through Syria and Arabia to Egypt and Africa, over a great part of which they are found, and eastward to India, Afghanistan, and Ceylon. A single species occurs at the Cape of Good Hope; and another is found in North America. We may commence by noticing this last species, as it not only makes the nearest approach to those of preceding families, especially the Muridæ, but differs from the rest of the Jerboas in characters of such importance, that Dr. Coues maintains its right to form a separate family (_Zapodidæ_). [Illustration: SKULL OF THE CAPE JUMPING HARE.]