254 



ELEMENTAR Y ANA TOMY. 



[LESS. 



milk dentition, refer in the same manner to the deciduous 



incisors, canines, and molars respectively. 



It need hardly be added that each tooth attains its full 



development within a limited time, after which it grows no 



more, and (with very rare exceptions) no third development 



ever replaces the fall of a tooth of the permanent dentition. 

 An acquaintance with human structure only, would give 



the student very little idea of the possibilities of development 



in the matter of " teeth." 



As to SITUATION, implantation, number, form, and use, and 



also as to succession and structure, the greatest diversities 



are to be found. 



21. Man agrees with the whole of his class in that he is 



only furnished with teeth upon the margins of the jaws. 



Some members of his class, however, are as completely 

 toothless as are Birds e.g. the Echid- 

 na, Pangolins, and Ant-eaters. 



In Reptiles we first become ac- 

 quainted with the fact that true teeth 

 may be developed not only from the 

 margins of the jaws, but also from the 

 palate, as we find to be the case in 

 the Iguana and in Serpents, some of 

 the teeth of which latter, as will shortly 

 be explained, present very remarkable 



peculiarities. When we descend to the Batrachians we some- 



FIG. 220. ANT-EATER. 



FIG. 221. OPEN MOUTH OF THE AMERICAN EFT PLETHODON, 



Showing the numerous para-sphenoidal teeth at the extreme back of the roof of 

 the mouth, together with a row of palatine teeth placed behind each posterior 

 nasal opening in the anterior part of the palate. 



times find teeth in two series not only in the upper jaw, but 

 also two series in the lower (as in Proteus and Meno- 



