BATRACHIANS. 187 



successional teeth in most mammalia ; for, exerting equal pressure 

 against the fang of the contiguous tooth, which, from being incompletely 

 formed, has a wide pulp-cavity with very thin walls, the nascent tooth 

 soon penetrates that cavity, and quits the recess in the alveolar plate 

 in which it was originally situated. Thus the stage of develop- 

 ment corresponding with the ' eruption' of the tooth in the mam- 

 malia is immediately followed by the ' inclusion' of the new 

 tooth in the pulp-cavity of its predecessor. Further details of the 

 development and succession of the teeth of the Crocodiles will be 

 given in the chapter appropriated to the description of the dental 

 system in that family ; but I may here observe that the rapid suc- 

 cession of tooth-germs, which stamps the impress of decay upon their 

 predecessors often before the growth of these is completed, though 

 common to many reptiles, is most strikingly manifested in the 

 Crocodiles, in which three and sometimes four generations of teeth, 

 sheathed one within the other, are contained in the same socket. 



CHAPTER II. 



TEETH OF BATRACHIANS. 



75. The variations which the dental system presents in the Batra- 

 chian order of Reptiles are more conspicuous in the number, situa- 

 tion, and structure of the teeth, than in their form or mode of attach- 

 ment. Certain Batrachians are edentulous, as the genus Hylaplesia, 

 among the tree-frogs, and the Bufonidos or family of Toads, some of 

 the species of Bombinator excepted. 



The teeth when present are generally numerous, simple, of small 

 and equal size, and close-set, either in a single row or aggregated 

 like the teeth of a rasp. 



It is not without interest to observe that a characteristic condi- 

 tion of the dental system in fishes, viz : the absence of teeth on the 

 superior maxillary bone, is continued in those genera of perenni- 



