448 ANATOMY OF VERTEBRATES. 



in tlie fifth and sixth, after which they gradually shorten to the 

 thirteenth, and continue of the same length to the last lumbar. 

 The metapophysis, commencing as a tuberosity above the diapo- 

 physis, passes gradually from that part to the outer side of the 

 prozygapophysis, which it finally attains in the seventeenth dorsal 

 vertebra, and continues in the same place throughout the lumbar 

 series. There are no anapophyses. The lumbar diapophyses are 

 long, broad, and in close juxtaposition ; the last presents an arti- 

 cular concavity adapted to a corresponding convexity on the fore 

 part of the diapophysis of the first sacral. 



The cervical vertebrae, though shorter than in Macrauchenia, 

 are longer than in other Perissodactyles, and rise Avith an arch to 

 support the head : the joints of the centrums are opisthocoelian. 

 In the third cervical the pleurapophysis is developed below the 

 arterial canal, and extends forward, outward, and downward. 

 The neural spine, u, has subsided to a low rough ridge. The 

 hypaj)ophysial ridge and tubercle, o, are well marked, as are also 

 the anterior convexity and posterior conca\dty of the centrum. 

 The inner surface of each neurapophysis is pierced by a small 

 canal in the same place and direction as that which transmits the 

 vertebral artery in Macrauchenia ; but the artery traverses the 

 base of the transverse process in the Horse, as in most other 

 mammals. In the axis the neural spine, k, is a strong but low 

 rugged ridge, which bifurcates posteriorly, and subsides upon the 

 zygapophyses. The diapophyses are short and triangular, with 

 their bases perforated by the vertebral artery. A strong ridge 

 on the under part of the centrum leads to the hypapophysis, h. 

 The posterior articular surface of the centrum is deeply exca- 

 vated. In the atlas, c, the anterior articular cavities do not meet 

 below : the diapophysial ridges, «, bend down, forming large con- 

 cavities : the vertebral artery twice pierces their base, which is 

 also traversed by a canal leading to the neural canal, anterior to 

 which the neural arch is perforated on each side. The hyj^apo- 

 physis developes a strong tubercle. 



In the skeleton of a Quagga {Equus Qiiagga^ I have observed 

 19 dorsal, 6 lumbar, 5 sacral, and 18 caudal vertebrae; in that of 

 a Zebra {Equus Zebra), 18 dorsal, 6 lumbar, 5 sacral, and 17 

 caudals ; whilst in the skeleton of an Ass {Eqiius Asinus), there 

 were 18 dorsal, 5 lumbar, 5 sacral, and 17 caudal vertebrae. The 

 sixth lumbar, fig. 299,/, becomes the first sacral by coalescence. 



B. Skull. — Some common characters of this part of the skeleton 

 in Perissodactyles are given at pp. 283, 284. In the Malayan 

 Tapir (^Tapirus indicus), the paroccipitals are compressed and 



