XIII PHYLTTM CHOTiDATA nn? 



of narrow caiulal vertebra^. There are never chevron bones iu the 

 caudal region of any existing Ungulate. 



Ill all the Uncjulata the sternebra:; are distinct. As a general rule 

 the prestennnn is narrow, sometimes (Horses and Tapirs) greatly 

 compressed hiterally, while the mesosternum is broad; but in the 

 Rhinoceros the mesosternum is no broader than tlie 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 aiiterior margin of the premaxilla; 

 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 



Fir;. 1154. — Side view of posterior parts of skull of IZorse (Equus cabaUus). AS. alisphenoid ; 

 £x 0. exoccipital ; l-'r. frontal ; g.f. glenoid fossa ; Ma, jugal ; oc. occipital condyle ; Pa. 

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

 p.t. posttympanic process; SO. supraoccipital; S<i. squamosal; th. tympano-hyal ; I'lj. 

 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 

 {pii). 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 premaxilloe 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 ; 



