TEETH OF REPTILES. 387 



Labyrinthodonts, which have a double row of teeth at the anterior 

 part of the lower jaw. 



The teeth of Reptiles, with few exceptions, present a simple 

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

 more or less acute. The cone varies in length and thickness ; its 

 transverse section is sometimes circular, but more commonly 

 elliptical or oval : and this modification of the cone may be traced 

 through every gradation, from the thick, round, canine-like tooth 

 of the Crocodile to the sabre-shaped fang of the Varanus, the 

 Megalosaur, and the Cladeiodon. 1 Sometimes, as in the fully 

 formed teeth of the Megalosaur, one of the margins of the com- 

 pressed crown of the tooth is trenchant, sometimes both are so ; 

 and these may be simply sharp-edged, as in the Varanus of Timor, 

 or finely serrated, as in the great Varanus, the Cladeiodon, and 

 the Megalosaur. 2 



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

 it may be polished, as in the Leiodon, or impressed with fine 

 lines, as in the Labyrinthodon, fig. 243, or raised into many 

 narrow ridges, as in the Pleiosaur and Polyptychodon, or 

 broken by a few broad ridges, as in the Iguanodon, fig. 273, or 

 grooved by a single longitudinal furrow, as in some Serpents, 

 fig. 269, c. 3 



The cone is longest and its summit sharpest in the Serpents : 

 from these may be traced, chiefly in the Lizard tribe, a pro- 

 gressive shortening, expansion of the base, and blunting of 

 the apex of the tooth, until the cone is reduced to a hemi- 

 spherical tubercle, or plate, as in Ct/clodus, fig. 272, and, in a more 

 remarkable degree, in the extinct shellfish-eating Saurian, called 

 Placodus. 4 



In the Pleiosaur the dental cone is three-sided, with one of the 

 angles rounded off. The posterior subcompressed teeth of the 

 Alligator, fig. 275, present a new modification of form ; here they 

 terminate in a mammillate summit, supported by a slightly con- 

 stricted neck. In the tooth of the Hylreosaur 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 extinct Cardiodon and the existing 

 Iguanas, the teeth of which are farther complicated by having 

 the margins notched. The great Iguanodon had the crown of 

 the tooth expanded both in length and breadth, and combining 



1 v. pi. 62 A, fig. 4. 3 Ib. pi. 65. CCL. vol. iv. figs. 209, 210. 



2 Ib. fig. Gc. 4 CXLIII. p. 169. 



c c 2 



