XL] ART1ODACTYLA. 199 



situated at the bottom of a deep fossa on the outer and 

 posterior side of the tympanic bulla. 



The mandible has a high ascending portion behind, a 

 transverse condyle, a very small coronoid process, and a flat 

 expanded angle, rounded posteriorly. 



The hyoid of the Pig is very different from that of most 

 other Ungulata. The basihyal is very small. The thyrohyals 

 are large, broad and flat, and ankylosed to the basihyal, but 

 with their extremities cartilaginous even in old animals. 

 There is a well-ossified ceratohyal, not ankylosed with the 

 basihyal, but the greater part of the anterior arch is a long 

 cartilaginous band, with one, or sometimes two, slender 

 ossifications near the middle part, representing the stylohyal. 



The skull of the Hippopotamus resembles that of the 

 Pig in many essential features, although its external form is 

 greatly modified. The brain cavity is very small, and the 

 face immensely developed. The orbits project outwards in 

 an almost tubular manner, and their margins are nearly, and 

 in some cases quite, complete posteriorly. The face is con- 

 tracted laterally in front of the orbits, and then expands 

 widely into a massive truncated muzzle, which supports the 

 great canine and upper incisor teeth. 



The anterior narial orifice is nearly circular ; it is bounded 

 by the extremities of the narrow but greatly elongated 

 nasals above, and laterally by the prominent, rounded, 

 rugged and massive premaxillae. At the anterior and lower 

 part of the orbit, the lacrymal is dilated into a great thin- 

 walled bony capsule, of such delicacy that it is nearly always 

 destroyed in the skeletons preserved in museums. This 

 opens into the nasal air-passages and has no connection 

 with the lacrymal apparatus. A similar but smaller dilatation 

 exists in many of the Pecora. 



