302 



PALAEONTOLOGY. 



size, it penetrates the large pulp-cavity of the previously 

 formed tooth either by a circular or semicircular perforation. 

 The size of the perforation in the tooth, and of the depression 

 in the jaw, proves them to have been in great part caused by 

 the soft matrix, which must have produced its effect by exciting 

 absorbent action, and not by mere mechanical force. The 

 resistance of the wall of the pulp-cavity having been thus 

 overcome, the growing tooth and its matrix recede from the 

 temporary alveolar depression, and sink into the substance 



of the pulp contained in the 

 cavity of the fully-formed tooth. 

 As the new tooth grows, 

 the pulp of the old one is re- 

 moved ; the old tooth itself is 

 next attacked, and the crown, 

 being undermined by the ab- 

 sorption of the inner surface of 

 its base, may be broken off by 

 a slight external force, when 

 the point of the new tooth is 

 exposed. The new tooth dis- 

 embarrasses itself of the cylin- 

 drical base of its predecessor 

 (fig. 104, a) with which it is 



Fig. 104. 



Section of jaw with teeth of the 

 Alligator. 



sheathed, by maintaining the excitement of the absorbent 

 process so long as the cement of the old fang retains any vital 

 connection with the periosteum of the socket ; but the frail 

 remains of the old cylinder, thus reduced, are sometimes lifted 

 out of the socket upon the crown of the new tooth (as in fig. 

 104, a), when they are speedily removed by the action of the 

 jaws. No sooner has the young tooth (fig. 105, b) penetrated the 

 interior of the old one (fig. 105, a) than another germ c, begins 

 to be developed in the same relative position as that in which 

 its predecessor began to rise, and the processes of succession 



