FORM. 181 



simple conical form, with the crown more or less curved, and the 

 apex more or less acute. The cone varies in length and thick- 

 ness : its transverse section is sometimes circular, but more com- 

 monly elliptical or oval, and this modification of the cone may be 

 traced through every gradation, from the thick round tooth of the 

 Crocodile (PI. 62 a, fig. 9) to the sabre- shaped fang of the Varanus, 

 the Megalosaur (ib. fig. 6) and the Cladeiodon (ib. fig. 4). Sometimes, 

 as in the fully-formed teeth of the Megalosaur, one of the margins of 

 the compressed crown of the tooth is trenchant, sometimes both are 

 so ; and these maybe simply sharp-edged, as in the Varanus of Timor 

 or finely serrated, as in the great Varanus (PI. 68, fig. 3.), the 

 Megalosaur and the Cladeiodon. 



The outer surface of the crown of the tooth is usually smooth ; 

 it may be polished, as in the Leiodon (PI. 72, fig. 1), or impressed 

 with fine lines, as in the Labyrinthodon (PI. 63, 1), or raised into 

 many narrow ridges as in the Pleiosaur and Polyptychodon (PI. 72, 

 figs. 3 and 4), or broken by a few broad ridges as in the Iguanodon 

 (PI. 70, fig. 1), or by a single longitudinal furrow, as in some ser- 

 pents. 



The cone is longest and its apex sharpest in the Serpents ; from 

 these may be traced, chiefly in the lizard tribe, a progressive shorten- 

 ing, expansion of the base and blunting of the summit, of the tooth, 

 until the cone is reduced to a hemispherical tubercle, or plate, as in 

 the Thorictes (PI. 66, fig. 6) and Cyclodus (ib. fig. 7). 



In the Pleiosaur, (PL 68, fig. 5), the dental cone is three sided, 

 with one of the angles rounded off". The posterior subcompressed 

 teeth of the alligator present a new modification of form ; here they 

 terminate in a mammilloid summit, supported by a slightly constricted 

 neck (PI. 75, fig. 3). In the tooth of the Hyleeosaur (PI. 62 a, fig. 8.) the 

 expanded summit is flattened, bent, and spear-shaped, with the 

 edges blunted. But the expansion of the crown is greatest in the 

 subcompressed teeth of the Iguanas (PI. 70, figs. 6 and 7 and 

 PI. 66, fig. 5), which are further complicated by having the 

 margins notched. The extinct Iguanodon has the crown of the tooth 

 expanded both in length and breadth, and combining marginal denta- 

 tions with longitudinal ridges, presents the most complicated external 



