108 BRITISH FOSSIL REPTILES. 



In the black Alligator of Guiana the first fourteen teeth of the lower jaw are 

 implanted in distinct sockets, the remaining posterior teeth are lodged close together 

 in a continuous groove, in which the divisions for sockets are faintly indicated 

 by vertical ridges, as in the jaws of the Ichthyosaurs. A thin compact floor of bone 

 separates this groove, and the sockets anterior to it, from the large cavity of the ramus 

 of the jaw ; it is pierced by blood-vessels for the supply of the pulps of the growing 

 teeth and the vascular dentiparous membrane which lines the alveolar cavities. 



The tooth-germ is developed from the membrane covering the angle between the 

 floor and the inner wall of the socket. It becomes in this situation completely en- 

 veloped by its capsule, and an enamel-organ is formed at the inner surface of the capsule 

 before the young tooth penetrates the interior of the pulp-cavity of its predecessor. 



The matrix of the young growing tooth affects, by its pressure, the inner wall of 

 the socket, and forms for itself a shallow recess ; at the same time it attacks the side 

 of the base of the contained tooth ; then, gaining a more extensive attachment by its 

 basis and increased size, it penetrates the large pulp-cavity of the previously formed 

 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 PI. 7.5, 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 

 partj 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. 



