214 ELEMENTARY BIOLOGY. [CHAP. 



c. The 8th vertebra: the concave facet at each end 

 of its centrum. 



d. The pth vertebra (sacrum): its centrum; convex 

 in front and with two convex tubercles behind : 

 its large strong transverse processes (sacral ribs) 

 directed somewhat backwards and expanded at 

 their ends. 



3. The posterior unsegmented portion of the vertebral 

 column (urostyle). 



a. An elongated rod-like bone, rather thicker to- 

 wards its anterior end, which bears two con- 

 cavities. 



b. Its posterior end ; tubular in the dried skeleton, 

 but in the fresh state filled with a cartilage 

 which projects beyond it posteriorly. 



c. The prominent ridge along its dorsal surface ; 

 wider and higher in front ; getting thinner and 

 gradually disappearing towards the posterior end. 



d. The small canal contained in the forepart of 

 the ridge. 



e. The two minute passages leading from the canal 

 to the outside of the ridge on each side. 



4. The vertebral column as a whole. 

 a. Its composition. 



a. Its anterior segmented part formed of the nine 



vertebrae. 

 P. Its posterior unsegmented part formed by the 



urostyle and nearly as long as the segmented 



part. 



