xiii PHYLUM CHORDATA 537 



of narrow caudal vertebras. 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 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. 1154) is elongated, especially in the facial region; the axis of 

 the skull, or the line from the anterior margin of the premaxillas 

 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 supraoccipital (SO.) has a prominent transverse 



To, 



Mtt, 



FIG. 1154. Side view of posterior parts of skull of Horse (Equus cdballus). AS. alisphenoid ; 

 Ex 0. exoccipital ; Fr. frontal ; g.f. glenoid fossa ; Ma, jugal ; oc. occipital condyle ; Pa. 

 parietal ; pp. paroccipital process ; Per. periotic ; p. g. postglenoid process of squamosal ; 

 p.t. posttympanic process; SO. supraoccipital ; Sq. squamosal; th. tympano-hyal ; Ty. 

 tympanic. (After Flower.) 



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

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

 running along the course of the sagittal suture. The exoccipital 

 develops a prominent, downwardly-directed, paroccipital 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 exocci- 

 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 premaxillaB in a great part of 

 their extent. The mandible has a large ascending ramus, and a 

 coronoid process which rises high above the level of the condyle ; 



