xm PHYLUM CHORDATA 521 



the Horses they are almost completely absent, and in the Elephants 

 they are all small, with the exception of the last. The number 

 of thoraco-lumbar vertebrse is nearly always nineteen in the 

 Artiodactyles, twenty-three in the Perissodactyles and in the 

 Proboscidea. Hyrax has a larger number of trunk-vertebrae 

 twenty-eight to thirty than any other terrestrial Mammal. The 

 transverse processes of the lumbar vertebrae are nearly always 

 elongated, flattened, and directed outwards, or outwards and slightly 

 forwards. Usually there is a single wide sacral vertebra united 

 with the ilia, ankylosed with which behind are a varying number 

 of narrow caudal vertebrae. There are never chevron bones in the 

 caudal region of any existing Ungulate. 



In all the Ungulata the sternebrae are distinct. As a general 

 rule the presternum is narrow, sometimes (Horses and Tapirs) 

 greatly compressed laterally, while the mesosternum is broad ; 

 but in the Rhinoceros the mesosternum is no broader than the 

 presternum. 



Among the Perissodactyle Ungulates the skull of the Horse 

 (Fig. 1166) is elongated, especially in the facial region ; the axis of 

 the skull, or the line from the anterior margin of the pre-maxillae 



FiQ. 1166. Side view of posterior parts of skull of Horse (Equus caballus). AS. alisphenoid ; 

 Ex O. exoccipital ; Fr. frontal ; gf. glenoicl fossa ; Ma, jugal ; oc. occipital condyle ; Pa. 

 parietal ; pp. par-occipital process ; Per. periotic ; pg. post-glenoid process of squamosal ; 

 pt. post-tympanic process ; SO. supra-occipital ; Sq. squamosal ; th. tympano-hyal ; Ty. 

 tympanic. (After Flower.) 



to the lower edge of the foramen magnum, is nearly straight, and 

 both the occipital plane and ethmoidal plane are nearly perpen- 

 dicular to it. The supra-occipital (SO.) has a prominent transverse 

 crest ; and in front of this the temporal ridges, which limit the 

 temporal fossa above, unite to form a median longitudinal sagittal 

 crest, running along the course of the sagittal suture. The ex- 

 occipital develops a prominent, downwardly-directed, par-occipital 

 process (pp). The tympanic (Ty.) is small and, with the periotic 

 (Per.), is only loosely connected with the neighbouring bones, 



