1876.] 



MR. E. R. ALSTON ON THE ORDER GLIRES. 



71 



Pedetince, 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 Chinchillidce. 



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 angu- 

 lar and dental portions. In the Caviidtc, 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 under- 

 stood by a comparison of figures 4 and 5. In the skull the infraorbital 

 opening is always large, oval or subtriangular, an interptervgoid 

 fissure is present, and the foramina of the base of the skull are pro- 

 portionally large, while the incisive foramina are small. The fron- 

 tals have no distinct postorbital processes (except in Chcetomys) ; 

 and the malar, which is rarely continued far forward, is not sup- 

 ported 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. Maram.ii.p. 149. 



