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Show 1876.] MR. E. R. ALSTON ON THE ORDER GLIRES. 71 Pedetina, of which the first is the most murine, and the second the most highly specialized, while the third shows more than superficial resemblances to the Chinchillida. The third section, Hystricomorpha, is characterized by the form of the mandible, combined with persistence of the fibula as a distinct bone throughout life. In the mandible the ascending ramus and coronoid process are low, and the angular portion does not spring from the lower edge of the bony covering of the lower incisor. In the great majority of forms in which that tooth is long, the angular portion springs from the outer side of its bony sheath, so that when viewed from below there is a longitudinal groove between the angular and dental portions. In the Caviida, in which the incisors are short, " the direction of the incisor is such that, were it prolonged Fig. 5. Mandible of Cavia aperea. backwards, the alveolus of the tooth and the angular portion of the jaw would hold the same relative positions" as in the other members of the section*. This difference in the form of jaw will be best understood by a comparison of figures 4 and 5. In the skull the infraorbital opening is always large, oval or subtriangular, an interpterygoid fissure is present, and the foramina of the base of the skull are proportionally large, while the incisive foramina are small. The frontals have no distinct postorbital processes (except in Chatomys) • and the malar, which is rarely continued far forward, is not supported below by a continuation of the maxillary zygomatic process. The clavicles are either perfect or imperfect; and one premolar is present above and below (except in Ctenodactylus). The upper lip is rarely cleft, the muffle is usually clad with very fine hairs, and the * Waterhouse, Nat. Hist. Mamm. ii. p. 149. |