The Common Squirrel may serve as an example of the whole genus _Sciurus_, which includes the ordinary Tree Squirrels, the species of which are very numerous, probably more than one hundred, and distributed over nearly all parts of the world. The species are most numerous in the warm Oriental regions, in India, and the countries and islands lying to the east of it, from which nearly fifty species have been recorded. The northern parts of the Old World only possess half a dozen species, but North America has about eighteen, many of which are considerably larger than the European Squirrel. The most striking of the North American species are the GREY SQUIRREL (_Sciurus carolinensis_) and the FOX SQUIRREL (_Sciurus niger_), both of which are abundant in the Atlantic States, and vary considerably in colour, presenting both grey and black individuals. [Illustration: BLACK FOX SQUIRREL.] Besides the ordinary Squirrels, a considerable number of other species are arboreal in their habits, and, indeed, even more strictly so than the true Squirrels. These are the Flying Squirrels, as they are called, which may be at once distinguished from the others by the presence of a large fold of skin, extending along the sides of the body, and including the limbs as far as the wrists and heels (_see_ figure on next page). In the case of the Common Squirrels, it is observed that in performing leaps of any considerable extent the limbs are stretched out, and the long, bushy tail extended, so as to give the animal as large a surface as possible; but in the Flying Squirrels, as in the Flying Lemur (Vol. I., p. 344), when the limbs are extended laterally the folds of skin (_patagia_) become tightly stretched, and form a regular parachute, which seems to give the animal essential support in its most extensive leaps. The extent of this membrane is increased by means of a sort of bony spur, which articulates with the wrist. The TAGUAN (_Pteromys petaurista_) is a large species, indeed, the largest of the whole family Sciuridæ. It measures about two feet long, and has a bushy tail of nearly equal length. Its ears are pointed, but not tufted, and its eyes are large and prominent. Its colour above is greyish-black, produced by a mixture of entirely black hairs with others having the tips greyish-white; beneath it is greyish-white. About the head and on the limbs the fur is tinged with brown or chestnut brown, and the lateral folds are sometimes of the latter colour, sometimes blackish-brown above and grey beneath. The tail is rounded in its form.