XIII.] THE FROG. 205 



between the occipital condyles : its arch, some- 

 times incompletely ossified in the region of the 

 spinous process, which is rudimentary ; posterior 

 zygapophyses only present: the large concave 

 anterior facets, partly on the arch and partly on 

 the centrum, with which the skull articulates. 



1. The 2nd, 4th, 5th, 6th and 7th vertebra : closely 

 resembling the third, the chief differences being 

 found in the varying size and direction of the 

 transverse processes, all of which are smaller than 

 those of the third vertebra. 



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

 of its centrum. 



d. The 9th 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 towards 

 its anterior end, which bears two concavities. 



1). 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. 



