The HETEROMYINÆ (forming the family Saccomyidæ of Dr. Coues, although he does not accept the genus _Saccomys_) are more slender and delicate in form than the Geomyinæ, and have the hind limbs and tail elongated, the former, indeed, being converted into leaping organs like those of the Jerboas and Kangaroos. The eyes and ears are larger, and the animals are in every respect adapted to life in the open, while the Geomyinæ, on the contrary, are subterranean in their habits. The hair in the present family is coarse and harsh, sometimes even spiny. In skeletal characters we find a similar alteration. The incisors are narrow; the skull is delicate, with its angles rounded off, and the mastoid bones form a considerable part of the roof of the cranial cavity; the zygomatic arches are slender; and, the lower root of the maxillary process being undeveloped, the infra-orbital opening is not defined. As in the Jerboas, the cervical vertebræ are sometimes anchylosed. Like the Geomyinæ, these animals are confined to America, and chiefly limited to the Southern United States and Central America, although some of the species occur as far north as the Columbia River and Hudson’s Bay, and one is found in Trinidad. By American writers they are called “Pocket Mice.” PHILLIPS’S POCKET MOUSE, also known as the Kangaroo Rat (_Dipodomys Phillipsii_), is one of the best known species of this group. It is an elegantly formed little creature, about four inches long, with a slender tail nearly six inches in length. Its colour above is mouse-brown, white beneath; the sides of the body have some white streaks, especially one from the ear towards the shoulder, and one on the thigh running towards the root of the tail; the tip of the tail is also white. This is a Californian species, but extends throughout the Pacific region of the United States. It is represented in the Rocky Mountains by a rather larger and stouter form, with smaller ears and a shorter tail (_Dipodomys Ordii_), which is generally regarded as distinct, but is placed by Dr. Coues as a sub-species. The habits of the species are comparatively little known, but they appear to live in the most desert places they can find, the barren spots on which the only plants that seem to flourish are the great mis-shapen cactuses. They dwell in holes under rocks and stones, from which they emerge at sunset, and hop about gaily after the fashion of little Kangaroos. The places in which these Pocket Mice are found are so bare of vegetation and destitute of water, that it is difficult to imagine how they contrive to exist. In all probability they pick up a scanty living in the shape of roots and grasses, especially seeds, carrying a supply for the day into their holes in their great cheek-pouches.