2i 4 ELEMENTARY BIOLOGY. [CHAP. 



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

 of its centrum. 



d. The Qth 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. 



<?. 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. 

 /3. Its posterior unsegmented part formed by the 



urostyle and nearly as long as the segmented 



part. 



