CROCODILIA. 27 



subdivide near the enamel, and terminate in fine and irregular branches, which 

 anastomose generally by the medium of cells. The dentinal tubes send off from both 

 sides, throughout their progress, minute branches into the intervening substance, and 

 terminate in the dentinal cells. These cells are subhexagonal, about -5-5-5- of an 

 inch in diameter, and are traversed by from ten to fourteen of the dentinal tubes ; they 

 are usually arranged in planes parallel with the periphery of the crown, near which 

 they are most conspicuous, and towards which their best defined outline is directed : 

 they combine with the parallel curvatures of the dentinal tubes to form the striae, 

 visible in sections of the teeth by the naked eye, which cause the stratified appearance 

 of the dentine as if it were composed of a succession of superimposed cones. The 

 diameter of the dentinal tube before the first bifurcation is 1 ., \ th of an inch, 

 both the trunks and bifurcations of the tubes have interspaces equal to four of their 

 respective diameters. 



The enamel viewed in a transverse section of the crown presents some delicate 

 striae parallel with its surface, whilst the appearance of fibres vertical to that surface 

 is only to be detected, and these faintly, on the fractured edge. It is a very compact 

 and dense substance ; the dark brownish tint is strongly marked in the middle of the 

 enamel when viewed by transmitted light. 



The cells with which the fine tubes of the basal cement communicate, are oblong, 

 about ^oVo th of an inch across their long axis, which is transverse to that of the tooth ; 

 the inter-communicating tubes, which radiate from the cells, giving them a stellate figure. 

 I have entered into these particulars of the microscopic texture of the teeth of the 

 Crocodile because it will be seen in the sequel that important modifications of 

 the dental tissues characterise some of the extinct Beptilia. 



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 

 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 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 



