8 4 ZOOLOGY OF THE VOYAGE OF THE BEAGLE 



Vertebral Column. 





Of this part of the skeleton of the Scelidothere, Mr. Darwin's specimen includes, 

 as is represented in Plate XX., the cervical, part of the dorsal, and the sacral series 

 of vertebrae in a more or less perfect condition. 



The cervical vertebrae present the ordinary mammalian number, seven, and are 

 free, or so articulated as to have permitted reciprocal movement upon each 

 other. Their transverse processes are perforated as usual for the vertebral 

 arteries. These processes in the atlas are remarkable for their great breadth, 

 length, and thickness ; and indicate the muscular forces which must have worked 

 the head upon the spine to have been very powerful. The axis is provided with a 

 robust ' processus dentatus,' having a base equal in breadth to the body of the 

 axis itself; and a smooth articular convexity on the side of the apex on which the 

 ring of the atlas rotated. The line of union between the axis and its characteristic 

 process, which here resembles the body of an abortive vertebra, is very distinct The 

 transverse processes of the vertebra dentata are comparatively feeble, but this con- 

 dition is amply compensated for by the great development of the spinous process. 

 (PI. XXIV. fig. 1.) This process is bent backwards at nearly a right angle, over- 

 laps with its reflected extremity the spine of the third cervical vertebra, and rests 

 by its base, on the under part of which are the posterior articular surfaces, upon 

 the broad and strong anterior oblique processes of the third vertebra. 



The third, fourth, fifth, and sixth cervical vertebrae have moderately developed 

 and pointed spinous processes : their transverse processes are broad, and extend 

 obliquely backwards, and slightly overlap each other. On the under part of the 

 transverse process of the sixth cervical vertebra there is the fractured base of what 

 1 conjecture to have been an expanded aliform plate, analogous to that observable 

 in the corresponding vertebra of the Orycterope. The seventh cervical vertebra 

 has part of the articular depression for the head of the first rib upon each side of 

 its body : the transverse process is feebly developed, but the spine is double the 

 height and size of those of the preceding vertebra. 



The spinous process of the first dorsal vertebra in like manner rises to twice the 

 height of the preceding spine of the seventh cervical, and preserves an equal antero- 

 posterior diameter from its base to its summit, which is thick and slightly bent 

 backwards : four or five succeeding dorsal vertebra give evidence of having been 

 surmounted by spines of equal height and strength. The transverse processes of 

 these dorsal vertebrae present bold concavities on their inferior part for the recep- 

 tion of the tubercles of the ribs, and they gradually ascend upon the base of the 

 spines as the vertebrae are placed further back, so as to increase the expansiveness 



