. An introduction to the osteology of the mammalia . nasal chambers and air sinuses. They arenarrowest in Bathyergus and its allies. The premaxillae are large, and lodge the great curvedincisor teeth,2 and always send a narrow prolongation back-wards by the side of the nasals to join the frontals. 1 See A. Milne-Edwards, Nouv. Archiv. du Museum, iii. 1867. 2 These teeth, though first developed in the gum covering the pre-maxilla, have their roots, when fully developed, in the maxilla. Thisdoes not invalidate their determination as incisors. N 2 i So THE SKULL. [CHAP. In the larger number of Ro
. An introduction to the osteology of the mammalia . nasal chambers and air sinuses. They arenarrowest in Bathyergus and its allies. The premaxillae are large, and lodge the great curvedincisor teeth,2 and always send a narrow prolongation back-wards by the side of the nasals to join the frontals. 1 See A. Milne-Edwards, Nouv. Archiv. du Museum, iii. 1867. 2 These teeth, though first developed in the gum covering the pre-maxilla, have their roots, when fully developed, in the maxilla. Thisdoes not invalidate their determination as incisors. N 2 i So THE SKULL. [CHAP. In the larger number of Rodents there is a great vacuityin the anterior or maxillary root of the zygoma of varying sizeand form, apparently an enormous dilatation of the infra-orbital foramen, and through which a portion of the massetermuscle passes (see Fig. 59). It is sometimes as large as theorbit tself, with which it communicates freely posteriorlyunderneath a vertical bar, formed by the maxilla in front, andby the lacrymal and malar behind. Its inferior boundary JflCr. Per ExO FIG. 5Q.—Side view of skull of Cape Jumping Hare (Pedetes caffer), f. Sq squamosal ;Pa parietal ; AS alisphenoid ; Fr frontal; OS orbitosphenoid ; L lacrymal;No. nasal; PMx premaxilla ; MX maxilla ; Ma malar ; Ty tympanic ; ExO ex-occipital ; Per points to the large supratympanic or mastoid bulla. is a slender zygoma-like horizontal bar, formed by the maxillaalone. In the Rats it is a vertical fissure dilated the Viscacha (Lagostomus), the true infraorbital foramenis separated from the large antorbital vacuity by a thin as-cending bony lamella. In Castor, Lepus, Bathyergus, Saurus,Arctomys and some others, the infraorbital foramen is of theusual size. In the Hares, the facial surface of the maxilla is curiouslyreticulated, x.] RODENTIA. 181 The zygoma is present in all, of various degrees of thick-ness, but always either straight or more or less curveddownwards, and usually not much arched outwards. Itsanterio
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