THE CHELONIAN SKULL. 



203 



but possess ribs which may not become antylosed with 

 the centra. Thus the tail and the neck are the only regions 

 of the spinal column of a Chelonian which are flexible. 



In the skulls of the Chelonia aU the bones, except the 

 mandible and the hyoidean arch, are immoveably united 

 together. 



In the occipital segment of the adult, the supra-occipital 

 is united with the epiotic, but the ex-occipital usually re- 

 mains perfectly distinct from the opisthotic. The basi- 



Fig. G5. 



Fig. 65.— External view of a section of the auditory region of the skull 

 in a Turtle {Chehne midns) :—f.o., fenestra ovalis ; f.r., fenestra ro- 

 tunda ; esc, asc, psc, external, anterior, and posterior semicircular 

 canals. 



sphenoid is large and distinct. The alisphenoidal region 

 remains unossified; but the large parietals send down a 

 prolongation on each side, which plays the part of an 

 alisphenoid. Neither the presphenoid nor the orbito- 

 sphenoids are represented by bone, but there are large 

 frontals. In the periotic capsule the large pro-otic and 

 the opisthotic (Cuvier's occipitale externe) remain distinct 

 bones, but the epiotic unites with the supra-occipital. 



The naso-ethmoidal cartilage largely persists; but it 



