522 ZOOLOGY SECT. 



being held in place mainly by a post-tympanic process developed 

 from the squamosal. A considerable part of the periotic 

 (mastoid portion) appears on the surface of the skull between this 

 and the exoccipital. The tympanic forms a tubular auditory 

 meatus, but is not expanded into a bulla. The glenoid fossa is 

 extended transversely, and is bounded behind by a post-glenoid 

 process. The orbit, which is relatively small, is completely sur- 

 rounded by bone. The nasals are large, and are separated from 

 the pre-maxillse in a great part of their extent. The mandible 

 has a large ascending ramus, and a coronoid process which rises 

 high above the level of the condyle ; the latter is elongated trans- 

 versely in co-ordination with the form of the glenoid cavity. 



The skull of the Rhinoceros differs from that of the Horse mainly 

 in the presence of large air-cells in the supra-occipital and parietal 

 bones, and in the orbit not being separated by bone from the 

 temporal fossa, except in the two-horned Asiatic species. The 

 post-glenoid process equals or exceeds the paroccipital ; the mastoid 

 does not appear on the surface, owing to the post-tympanic process 

 of the squamosal extending backwards to articulate with the 

 exoccipital and concealing it from view. 



The skull of the Tapirs resembles that of the Rhinoceros in 

 most respects. As in the latter, the orbits are not completely 

 bounded by bone behind. The nasal openings are very large, and 

 extend backwards above the orbits, separated from them only by 

 a thin plate. The nasals are very prominent, and the inferior 

 and lateral boundaries of the nasal apertures are formed entirely 

 by the maxillae. There are large post-glenoidal and post-tympanic 

 processes ; the latter is united with the paroccipital process. The 

 mandible differs from that of the other Perissodactyles chiefly in 

 the prominent incurved angle. 



In the Ruminant Artiodactyles (Fig. 1167) the facial region is 

 more or less bent downwards on the basi-cranial axis, and while the 

 occipital plane is nearly perpendicular to the latter, the ethmoidal 

 plane is nearly horizontal. There are prominent paroccipital 

 processes (pp). The tympanic (Ty), which may or may not be 

 ankylosed with the periotic, forms a tubular auditory meatus and 

 sometimes a distinct bulla. The mastoid appears for a short space 

 on the surface, between the squamosal and the exoccipital. The 

 frontals usually bear a pair of processes, more or less prominent, for 

 the support of the horns, and between these a transverse ridge 

 frequently extends. The orbit is completely encircled by bone, 

 and has a prominent margin. The nasals are elongated and the 

 premaxillae slender. The condyle of the mandible is broad and 

 flat ; the horizontal ramus usually rather slender, and expanded in 

 front for the lodgment of the incisors. 



In the Pigs, as in the Ruminants, the facial region is bent 

 downwards. There is a prominent transverse occipital crest at 



