538 ZOOLOGY sect. 



the latter is elongated transversely in co-ordination with the form 

 of the glenoid cavity. 



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

 in the presence of large air-cells in the supraoccipital and pariettU 

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

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

 postglenoid 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 ex- 

 occipital 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 

 maxilUe. There are large post-glenoidal and post-tympanic pro- 

 cesses ; 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 Artiodaetyles (Fig. 1155) 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 ma,}^ 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 

 premaxillse 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 the 

 iunction of the supraoccipital and parietals ; but the temporal 

 ridges do not meet in the middle to form a sagittal crest such as 

 occurs in the skull of the Horse. There are prominent paroccipital 

 processes. There is a large, but compressed, bulla tympani ; the 

 auditory meatus is very long, directed upwards and outwards, and 

 is surrounded by the post-glenoidal and post-tympanic processes, 

 which are in contact with one another. The mastoid is rudi- 

 mentary, and does not appear on the outer surface of the skull. 

 The frontal develops a short postorbital process ; but this does 

 not meet the zygoma, so that the bony margin of the orbit is 



