TEETH OF EEPTILES. 405 



of the cavity : these are termed the ' Thecodont ' Lacertians : their 

 dental character is seen in the oldest known of all Saurians, viz. the 

 (Protorosaurus of the Thuringian copper-slates), and the Palao- 

 saurus of the dolomitic conolomerates near Bristol. The com- 

 pressed Varanian form of tooth, with trenchant and finely dentated 

 margins, which characterised these ancient Lizards, is continued 

 in the comparatively more recent and gigantic species called Mega- 

 losaurus. In this terrestrial carnivorous Reptile the teeth, when 

 first protruded above the gum, presented a double cutting edge of 

 serrated enamel; the position and line of action were nearly vertical, 

 and, like the two-edged point of a sabre, the teeth cut equally on 

 each side. As the tooth advanced in growth it became curved 

 backward, in the form of a pruning-knife, and the edge of serrated 

 enamel was continued downward to the base of the inner and 

 cutting side of the tooth, whilst on the outer side a similar edge 

 descended but a short distance from the point, and the convex 

 portion of the tooth became blunt and thick, as the back of a, 

 knife is made thick for the purpose of producing strength. The 

 strength of the tooth was further increased by the expansion of 

 its side. ' In a tooth thus formed for cutting;; alone; its concave 

 edge, each movement of the jaw combined the power of the knife 

 and saw : whilst the apex, in making the first incision, acted like 

 the two-edged point of a sabre. The backward curvature of the 

 full-grown teeth enabled them to retain, like barbs, the prey 

 which they had penetrated. In these adaptations we see con- 

 trivances which human ingenuity has also adopted in the prepara- 

 tion of various instruments of art.' ' 



The teeth of the Megalosaur consist of a central body of 

 dentine, with an investment of enamel upon the crown, and of 

 cement over all, but thickest upon the fang. The marginal 

 serrations are formed almost entirely by the enamel. The remains 

 of the dentinal pulp are converted into a coarse bone in the com- 

 pletely formed tooth. 



In most Pterodactyles the teeth are of one kind, few and far 

 apart, fig. Ill, with long, slender, compressed, slightly recurved, 

 pointed crowns ; but some, from the more ancient secondary 

 deposits, show, behind a few teeth of the above prehensile 

 character, a close-set row of small lancet-shaped teeth: such 

 modification characterises the genus Dimorphodon. 



The teeth of the Ichthyosaur have a simple more or less 

 acutely conical form, with a long and, usually, expanded or 



1 Buckland, Bridgewater Treatise, vol. i. p. 237. 



