26 FOSSIL REPTILIA OP THE LONDON CLAY. 



The teeth. — The most readily recognisable character by which the existing 

 Crocodilians are classified and grouped in appropriate genera, are derived from 

 modifications of the dental system. 



. . 18 — 18 22—22 



In the Caimans (genus Alligator) the teeth vary in number from to : 



18 — 18 22 — 22 



the fourth tooth of the lower jaw is received into a cavity of the alveolar surface of the 

 upper jaw, where it is concealed when the mouth is shut. In T. VIII, fig. 2, these 

 pits are shown behind the last premaxillary tooth e, in an Eocene Alligator from 

 Hordwell. In old individuals of the existing species of Alligator, the upper jaw is 

 perforated by the large inferior teeth in question, and the fossa? are converted into 

 foramina. 



In the Crocodiles (genus Crocodilus) the fourth tooth in the lower jaw is received 

 into a notch excavated in the side of the alveolar border of the upper jaw, as in fig. 1, 

 T. VIII, behind the tooth e, and is visible externally when the mouth is closed, as in 

 T. VII, fig. 1. In most Crocodiles, also, the first tooth in the lower jaw perforates 

 the premaxillary bone when the mouth is closed, as in T. I, between the teeth 

 marked a and b. 



In the two preceding genera the alveolar borders of the jaw have an uneven or 

 wavy contour, and the teeth are of an unequal size. 



In the Gavials, (genus Gavialis) the teeth are nearly equal in size and similar in 

 form in both jaws, and the first as well as the fourth tooth in the lower jaw, passes 

 into a groove in the margin of the upper jaw when the mouth is closed, T. XL 



In the Alligators and Crocodiles the teeth are more unequal in size, and less regular 

 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 T. VII and T. Ill, fig. 6, 

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

 T. V, 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 



