The Skeleton 57 



and neural arch, is a heavy bone shaped like a 

 truncated pyramid (4); the base of the pyra- 

 mid is ankylosed with the iliui". These bones 

 seem to be much thickened transverse processes, 

 but since they are not completely fused with 

 their respective vertebrae and are said to ossify 

 separately they should probably be called sacral 

 ribs. The two sacral vertebra? do not seem to 

 be any more closely united than are any other two 

 vertebrse. 



The Caudal VertebrcB (Fig. 16, C). These are 

 characterized by the entire absence of ribs, and 

 by the presence on all but the first and the last 

 four or five of V-shaped chevron bones. The 

 first ten or twelve of these chevron bones articulate 

 chiefly with the postero-ventral ends of the centra, 

 but they also articulate with the antero-ventral 

 ends of the vertebra behind themselves ; and as they 

 are followed caudad the}^ seem to lie directly 

 below the intervertebral regions and to articulate 

 equally with the vertebrce before and behind. 

 The chevron bones gradually diminish in size 

 from before back. The neural processes of the 

 first four or five caudals are broad, like those of the 

 more anterior regions, but caudad to this point they 

 become narrower and more pointed, though they 

 retain the same height until about the last ten or 

 twelve vertebrse. Towards the tip of the tail the 

 dorsal spines diminish in height and finally dis- 

 appear. The transverse processes of the fi^rst five 



