CROCODILIANS. 293 



Development. — In the black Alligator of Guiana the first four- 

 teen teeth in 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 Ichthyosaurus. (1) 



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 enveloped 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 aff"ects, by its pressure, 

 the inner wall of the socket, as shown in fig. 4, PL 75, and forms for 

 itself a shallow recess : at the same time it attacks the side of the 

 base of the contained tooth, as shown in fig. 3 : 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 semi-circular 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 fig. 4, the tooth a 

 having been displaced and turned round to show the eflfects 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, which must have produced its 

 effect by exciting vital action of the absorbents, 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 



cn PI. 75, fig. 3. 



