XIII 



PHYLUM CHORDATA 



539 



incomplete behind. The facial region as a whole is elongated and 

 laterally compressed. The nasals are long and narrow, and the 

 premaxillse send backwards long processes on each side of them. 

 A peculiar bone the pre-nasal is developed in the nasal septum. 

 The condyle of the mandible is transversely elongated ; the coronoid 

 process very small. 



The skull of the Hippopotamus differs from that of the Pig 

 mainly in the proportions of the various parts. The cranial cavity 

 is relatively small, and the face large. The orbits are almost 



FIG. 1155. Section of skull of Sheep .(Ovis aries). AS. alisphenoid ; bh, basihyal ; BO. 

 basioccipital ; S. basisphenoid ; cd. condyle ; ch. ceratohyal ; cp. coronoid process ; cli. 

 epihyal ; EO. exoccipital ; ET. ethmo-turbinal ; Fr. frontal ; ME. mesethmoid ; M T. maxil- 

 lary turbinal ; MX. maxilla ; Na. nasal ; OS. orbito-sphenoid ; Pa. parietal ; PL palatine ; 

 Per. periotic; P. MX. premaxilla ; P.S. pre-spbenoid ; Pt. "pterygoid" ; s. h. stylohyal ; SO. 

 supraoccipital ; pp. paroccipital process ; th. thyrohyal ; ty. tympanic ; Vo, vomer. (After 

 Flower. ) 



tubular, and are almost, or quite, encircled by bone. The face is 

 laterally contracted in front of the orbit and again expands 

 anteriorly. The mandible is extremely massive ; anteriorly the 

 symphysial portion is greatly expanded to support the large incisor 

 and canine teeth. 



In the Hyracoidea (Fig. 1156) the skull shows affinities with 

 Rodents and also with Perissodactyles. The zygomatic arch is stout : 

 it is formed mainly by the jugal (/%), which forms part of the 

 glenoid fossa. The postorbital processes may meet to bound the 



