XIII 



PHYLUM CHORDATA 



541 



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

 and postglenoidal 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 into a spout-like process at the 

 symphysis. 



In the Ungulata vera the scapula (Fig. 1158) 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, as in some other Mammals, 

 the vertebral portion of the scapula re- 

 mains cartilaginous, forming the so- 

 called supra-Scapular cartilage (ss). In 

 Pigs and some Perissodactyles, though 

 there is no acromion, there is a tri- 

 angular process about the middle of the 

 spine. 



The humerus is short and stout, the 

 radius always well developed, the ulna 

 in some (Pigs, Hippopotami, Tapirs, and 

 Rhinoceroses), well developed, in others 

 (the Horses and the Ruminants), incom- 

 plete. 



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. 1159) the fifth of 



the fore-foot is developed as well. The Horses (Fig. 1160) present 

 the greatest reduction in the number of the digits observable in 

 any Mammal, the third being the Only functional digit in each 

 foot. Its elongated metacarpal or metatarsal (sometimes called 

 the cannon lone) has in apposition with it laterally a pair of splint- 

 like vestiges which represent the metacarpals or metatarsal s of 

 the second and fourth digits. In the Artiodactyla, on the other 

 hand, the third and fourth digits form a symmetrical pair. In 



VOL. II L L 



FIG. 1158.- Right scapula of Red 

 Deer (Cervus elaphus). a. 

 acromion ; a/. prescapular 

 fossa ; c. vestigial coracoid pro- 

 cess ; gc. glenoid cavity ; pf. 

 post-scapular fossa ; sp. spine ; 

 ss. imperfectly ossified supra- 

 scapular portion. (After 

 Flower.) 



