The jerboas constitute, in the family of _dipodidæ_, a tribe composed of several species, which are found in eastern and central Europe, Asia, and Africa. The jerbilles, owing to the similarity of name, are often confounded with the jerboas; but the only things they have in common are a certain conformity of habits, and a nearly equal aptitude for leaping. Otherwise, their organization rather resembles that of the rat, along with which it is classed by zoologists. Their hind-legs are much shorter than those of the jerboa, and their tail is garnished with but a few short, stiff hairs. Like the jerboas, they inhabit the sandy wildernesses of Africa, Asia, and eastern Europe. [Illustration: JERBOAS ATTACKED BY A HORNED VIPER.] These small animals, exclusively frugivorous and graminivorous, seem able, in the solitary places where they make their retreats, to multiply themselves _ad infinitum_; but, while a great number perish through famine, they are also decimated by a host of enemies in the reptiles of the Desert, and especially by the terrible horned viper, or _cerastes_, and a great saurian, intermediate between the lizard and crocodile--the "varan of the Desert." The horned viper (_vipera cerastes_) is thus named on account of the two horns or protuberances on its forehead, which give it a physiognomy more hideous, perhaps, than that of any of its congeners. It attains the length of two to three feet. Its head is depressed, very obtuse, swollen behind the eyes, and, so to speak, truncated in front. Its body, cased in shells of a tawny-like yellow, marked with brown spots, blends curiously with the sand, half-buried in which it lurks to surprise its prey or escape from its enemies. The cerastes frequents the deserts of Lybia, Arabia, the Sahara, and the valley of the Nile. Its bite is exceedingly dangerous. [Illustration: 1. Varan of the Nile. 2. Varan of the Desert.] The _varans_, or _monitors_, called also _tupinambis_ by the ancient naturalists, form a genus represented in tropical climes by several species of great size. English writers commonly designate them monitors, the French sauvegardes, because they frequent the haunts of crocodiles and alligators, and give warning of their approach by a whistling sound. Two species belong to Africa: one, aquatic, the varan of the Nile (_varanus dracæna_); the other, sand-burrowing, the varan of the Desert (_varanus sunius_, or _arenarius_), called by the Arabs _onaran-el-ard_. Their usual size is from three feet to three feet four inches. The varan of the Nile wears an armour of alternately green and black scales. Its congener exhibits a mixture of brown and yellow, more suitable to its sandy lairs. It is rare in the Sahara, but common enough in the deserts of Egypt, Syria, and Nubia.