CROCODILIA. 107 



in arrangement, and more diversified in form than in the Gavials : witness the strong 

 thick conical laniary teeth at the fore part of the jaw, as shown in PI. 2 A, figs. 3 and 6, 

 as contrasted with the blunt mammillate summits of the posterior teeth, as shown in 

 Pi. 3 A, fig. 12. The teeth of the Gavial are subequal, most of them are long, slender, 

 pointed, subcompressed from before backwards, with a trenchant edge on the right 

 and left sides, between which a few faint longitudinal ridges traverse the basal part of 

 the enamelled crown. 



The teeth of both the existing and extinct Crocodilian reptiles consist of a body of 

 compact dentine forming a crown covered by a coat of enamel, and a root invested by 

 a moderately thick layer of cement. The root slightly enlarges, or maintains the same 

 breadth to its base, which is deeply excavated by a conical pulp-cavity extending into 

 the crown, and is commonly either perforated or notched at its concave or inner side. 



The dentinal tubes in the crown of a fully-developed tooth form short curvatures at 

 their commencement at the surface of the pulp-cavity, and then proceed nearly straight 

 to the periphery of the crown ; they very soon bifurcate, the divisions slightly 

 diverging ; then continuing their course with gentle parallel undulations, they 

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

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

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

 terminate in the dentinal cells. These cells are subhexagonal, about -^^ 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 T^Tnrth 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 -yijVirth 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 RejMlia. 



