XIII 



PHYLUM CHORDATA 



509 



extensive air-cells. The sutures are early obliterated. Paroccipital 

 and post-glenoidal processes are absent. The tympanic forms a 

 large, rounded auditory bulla ; but the external auditory meatus 

 is bounded chiefly by the post-tympanic process of the squamosal. 

 The mastoid portion of the periotic does not appear on the 

 surface. The orbit is not completely separated by bone from 

 the temporal fossa. The nasal aperture is situated far back, 

 and looks upwards and forwards almost as in the skull of 

 some of the Cetacea. The chief characteristic of the mandible 

 is its prolongation forwards with a spout-like process at the 

 symphysis. 



In the Ungulata vera the scapula (Fig. 1089) is never very broad ; 

 the spine is usually near the middle. Neither the acromion nor the 

 coracoid process is very prominent ; some- 

 times, as in the Horse, the former is ab- 

 sent. A clavicle is never present. In the 

 Ruminants the vertebral portion of the 

 scapula remains cartilaginous, forming 

 the so-called supra- scapular cartilage (ss). 

 In Pigs and some Perissodactyles, though 

 there is no acromion, there is a triangular 

 process about the middle of the spine. 



The humerus is short and stout, the 

 radius is always well developed, the 

 ulna is in some (Pigs, Hippopotami, 

 Tapirs, and Rhinoceroses) well developed, 

 in others (the Horses and the Rumin- 

 ants) it is incomplete. 



The first digit is always absent. 

 There is never a centrale. The trape- 

 zium and magnum unite in most of the 

 Ruminants. 



In the Perissodactyla the third digit 

 in both the fore and hind foot is sym- 

 metrical in itself. In the Rhinoceroses 

 the second and fourth are also present, 

 and in the Tapirs (Fig. 1090) the fifth 

 of the fore-foot is developed as well. 

 The Horses (Fig. 1091) present the 

 greatest reduction in the number of the 



digits observable in any Mammal, the third being the onlj 

 functional digit in each foot. Its elongated metacarpal or meta- 

 tarsal (cannon bone) has in apposition with it laterally a pair of 

 splint-like vestiges which represent the metacarpals or meta- 

 tarsals of the second and fourth digits. In the Artiodactyla, 

 on the other hand, the third and fourth digits form a symmetrical 

 pair. In the Ruminant Artiodactyles (Fig. 1093) the metacarpals 



FIG. 1089. Right scapula of Red 

 Deer (Cf>-cus elaphus). a. 

 acromion ; af. prescapular 

 fossa ; c. vestigial coracoid pro- 

 cess ; yc. gleiioid cavity ; pf. 

 post-scapular fossa ; sp. spine ; 

 ss. imperfectly ossified supra- 

 scapular portion. (After 

 Flower.) 



