116 A MANUAL OF DENTAL ANATOMY. 



course of that slow rotation of the whole tooth-bearing 

 mucous membrane over the border of the jaw, which is 

 constantly going on. 



In the section figured there are four teeth advanced in 

 calcification, while beneath them are four tooth germs in 

 earlier stages ; of the former two only are fully protruded 

 through the epithelium, the third being in part covered 

 in ; the remaining teeth are altogether beneath the surface 

 of the epithelium, and therefore shut off from the cavity of 

 the mouth, if the soft parts be all in situ. 



All the teeth not fully calcified are covered in and pro- 

 tected by a reflexion upwards of the mucous membrane (c in 

 the figure), which serves to protect them during their calci- 

 fication. 



But although this may be termed a fold reflected upwards, it is 

 not, as was supposed by Professor Owen, a free flap, detached from 

 the opposite surface on which the teeth are developing ; there is no 

 deep open fissure or pouch running round inside the jaw, as would 

 in that case exist, and the epithelium does not pass down on the 

 one side to the bottom of such fissure, and then ascend upon the 

 other as a distinct layer. Although the fold is very easily torn 

 away from the tooth germs which it covers in, yet in the natural 

 condition it is attached, and there is no breach of surface ; the 

 epithelium passing across from the jaw to cover it is well seen in 

 the figure, in which the epithelial layer is represented as broken 

 just at the point (between the third and fourth teeth) where it 

 leaves the jaw to cross over on to the surface of the flap. 



The conditions met with in the Elasmobranch fishes are 

 peculiarly favourable for the determination of the homolo- 

 gies of the several parts of the tooth germ and of the formed 

 tooth ( 1 ). At the base of the jaw, where the youngest 

 tooth germs are to be found, the tissue whence the dentine 

 papillae arise blends insensibly with that making up the 

 substance of the thecal fold on the one hand, and on the 



(!) Compare the description of the placoid dermal spine (page 2). 



