The Squirrel-like Rodents have four molars on each side in the lower jaw, and either four or five in the upper. When the latter number is present, the foremost tooth is smaller than the rest. The fibula remains as a distinct bone through life, and is usually quite free, although sometimes attached to the tibia at the extremity. The upper lip is usually cleft, the muffle is small and naked, and the nostrils are comma-shaped, with the rounded part above. The zygomatic arch is formed chiefly by the process of the malar bone, which is not supported below by a continuation of the zygomatic process of the maxillary. The collar-bones are perfect. The tail is usually cylindrical and hairy. The Mouse-like Rodents agree with the preceding in the characters of the upper lip, muffle, and nostrils, but they have the tibia and fibula completely united for at least the last third of their length. The zygomatic arch is slender, and the malar process rarely extends so far forward as in the preceding group and is generally supported below by a continuation of the maxillary process. The collar-bones are perfect, except in one very small family; and the tail is cylindrical, and although sometimes hairy, more commonly covered with scales arranged in rings. The number of molar teeth in this section varies from three to six[51] on each side in each jaw, but three is the most usual number. [Illustration: TEETH OF THE TAGUAN.] The Porcupine-like Rodents, with one exception, have four molars on each side in both jaws; the fibula distinct throughout life; the upper lip rarely cleft; the muffle clad with a velvety coat of fine hairs; and the nostrils either S-shaped or straight. The zygomatic arch is stout, and the malar process does not advance far forward, nor is it supported below by the maxillary process. SECTION I.--(_SCIUROMORPHA._) SQUIRREL-LIKE RODENTS. FAMILY I.--SCIURIDÆ. This first family, which includes the true Squirrels and the Marmots, is distinguished from the rest of the section by the possession of five rooted molars on each side of the upper jaw (_see_ figure of the teeth on p. 85), the first being very small and sometimes deciduous, and four molars on each side of the lower jaw, and by the presence on the skull and zygomatic arch of small processes, indicating the posterior boundary of the orbits (_see_ figure of the skull on p. 82). The molars are tubercular, at least at first; but the summits of the tubercles are generally more or less worn down as the animal increases in age. [Illustration: COMMON SQUIRREL.]