28 FOSSIL REPTILIA OF THE LONDON CLAY. 



tooth, either by a circular or semicircular perforation. The size of the calcified part 

 of the tooth matrix which has produced the corresponding absorption of the previously 

 formed tooth on the one side, and of the alveolar process on the other, is represented 

 in the second exposed alveolus of the portion of jaw figured in PL 75, fig. 4, of my 

 ' Odontography,' the tooth marked a in that figure, having been displaced and turned 

 round to show the effects of the stimulus of the pressure. 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, exciting dissolution and 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 removed ; the old 

 tooth itself is next attacked, and the crown being undermined by the absorption 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 disembarrasses itself of the cylindrical base of its predecessor, with 

 which it is sheathed, by maintaining the excitement of the absorbent process so long 

 as the cement of the old fang retains any vital connexion with the periosteum of the 

 socket ; but the frail remains of the old cylinder, thus reduced, are sometimes lifted off 

 the socket upon the crown of the new tooth, when they are speedily removed by the 

 action of the jaws. This is, however, the only part of the process which is immediately 

 produced by mechanical force : an attentive observation of the more important pre- 

 vious stages of growth, teaches that the pressure of the growing tooth operates upon 

 the one to be displaced only through the medium of the vital dissolvent and absorbent 

 action which it has excited. 



Most of the stages in the development and succession of the teeth of the Crocodiles 

 are described by Cuvier* with his wonted clearness and accuracy ; but the mechanical 

 explanation of the expulsion of the old tooth, which Cuvier adopts from M. Tenon, is 

 opposed by the disproportion of the hard part of the new tooth to the vacuity in the 

 walls of the old one, and by the fact that the matter impressing — viz. the uncalcified 

 part of the tooth-matrix — is less dense than the part impressed. 



No sooner has the young tooth penetrated the interior of the old one, than another 

 germ begins to be developed from the angle between the base of the young tooth and 

 the inner alveolar process, or in the same relative position as that in which its imme- 

 diate predecessor began to rise, and the processes of succession and displacement 

 are carried on, uninterruptedly, throughout the long life of these cold-blooded 

 carnivorous reptiles. 



From the period of exclusion from the egg, the teeth of the crocodile succeed each 

 other in the vertical direction ; none are added from behind forwards, like the true 



* Op. cit., pp. 90-3. 



