SKELETON OF PERISSODACTYLA. 445 



are articulated to one another. Only tlie first two of the anchy- 

 losed sacrals afford articular surfaces to the ilia. Sometimes a 

 coalesced caudal adds a seventh vertebra to the sacrum. The 

 atlas has a recurved hypapophysis : its articular cups are deep ; 

 the base of the transverse process is tmce perforated by the 

 vertebral artery, the anterior hole opening upon the groove 

 which leads to the foramen in the neural arch common to the 

 vertebral artery and the first spinal nerve. 



In the Rhinoceros, fig. 165, the vertebral formula is — 7 cer- 

 vical c, 19 dorsal D, 3 lumbar, 4 sacral, and 22 caudal. In the 

 atlas, the hypapophysis developes a process from the lower part 

 of the anterior surface. The neural arch is perforated trans- 

 versely by the vertebral artery. In the axis the centrum supports 

 a simple diapophysis, inclining downward and backward. The 

 neural spine is thick, short, tuberculated, and divided by a deep 

 and broad groove into tAvo : the upper part of the spine is pro- 

 longed obliquely upward, giving the whole a trifid character. 

 The pleurapophyses, from the fourth to the sixth cervical ver- 

 tebrae inclusive, have the form of broad subquadrate plates : in 

 the seventh the diapophysis only is developed, and the transverse 

 process is consequently imperforate. The spine of this vertebra 

 suddenly acquires great increase of length, which continues more 

 gradually to the second and third dorsals, beyond which the 

 spines shorten, but gain in antero-posterior extent to the eleventh 

 dorsal, beyond which they continue of the same size, shape, and 

 inclination to the lumbar region. A metapophysis rises in the 

 fourth dorsal from the back of the diapophysis, from which it 

 becomes distinct in the sixteenth dorsal. The diapophysis, which 

 gradually subsides in the dorsal, reappears suddenly in the first 

 lumbar : it becomes shorter in the second ; and still more so in 

 the third, in which it is very broad. The lower edge of the dia- 

 pophysis of the second lumbar articulates witli the upper edge of 

 the diapophysis of the third, and the third articulates in the same 

 manner with the first vertebra of the sacrum. The metapophyses 

 are distinct, and are situated on the anterior zygapophyses in the 

 first two lumbars : in the last they have become rudimental, and 

 almost obsolete. The centrum is strongly convex anteriorly, and 

 concave behind, in the cervical vertebra? ; the dorsals are opis- 

 thocoelian in a less degree. 



The ribs are slender in proportion to their length, and more 

 curved than in the Elephant. In the first rib the tubercle is 

 large, with a corresponding articular surface : both this and the 

 second are almost straight, become expanded distally, and have no 



