xiii PHYLUM CHORDATA 505 



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

 region of any existing Ungulate. 



In all the Ungulata the sternebrse are distinct. As a general rule 

 the pre-sternum is narrow, sometimes (Horses and Tapirs) greatly 

 compressed laterally, while the meso-sternum is broad ; but in the 

 Rhinoceros the meso-sternum is no broader than the pre-sternum. 



Among the Perissodactyle Ungulates the skull of the Horse 

 (Fig. 1085) is elongated, especially in the facial region ; the axis of 

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

 to the lower edge of the foramen magnurn, 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 tem- 



FIG. 1085. Side view of posterior parts of skull of Horse (Equus caballus). AS. alisphenoid ; 

 Fr. frontal ; g. f. glenoid fossa ; Ma, jugal ; oc. occipital condyle ; Pa. parietal ; pp. par- 

 occipital process ; Per. periotic ; p. g. post-glenoid process of squamosal ; p.t. post- tympanic 

 process ; SO. supra-occipital ; Sq. squamosal ; t. h. tympanic hyal ; ty. tympanic. (After 

 Flower.) 



poral fossa above, unite to form a median longitudinal sagittal crest, 

 running along the course of the sagittal sutere. The ex-occipital 

 develops 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, 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 ex-occi- 

 pital. 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 surrounded 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 



