TEETH OF BEPTILES. 405 



of the cavity : tliesc are termed the ' Thecodont ' Lacertiaiis : their 

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

 (Frotnrnsaurv,s of tlie Thuringian copper-slates), and the Palao- 

 ■sdiirns of the dolomitic conglomerates near Bristol. The com- 

 pressed Varan ian form of tooth, with trenchant and finely dentated 

 margins, wliicli characterised these ancient Lizards, is continued 

 in the comparatively more recent and gigantic species called Mer/a- 

 /(isiinrua. In this terrestrial carnivorous Reptile tlie teeth, when 

 first protruded above the gum, ]}resentcd a doidDle cutting edge of 

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

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

 each side. As the tooth advanced in jirowtli it l)ecame curved 

 jjackward, in the form of a pruning-knife, and tlie 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 

 ]»n-tion of the tooth became Ijlunt and thick, as the back of a 

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

 strengtli of the tooth was further increased by tlie expansion of 

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

 edge, each movement of the jaw comliined tlie power of the knife 

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

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

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

 wliich tliey liad penetrated. In these adaptations we see con- 

 trivances wliicli human ingenuity has also adopted in the prepara- 

 tion of various instrimients of art.'' 



The teeth of tlie Mef/alosam- 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. Tlie marginal 

 serrations are formed almost entirely liy the enamel. The remains 

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

 j)letely formed tooth. 



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

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

 pointed crowns ; l)ut some, from the more ancient secondary 

 deposits, show, behind a few teeth of tlie above iireliensile 

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

 modification characterises the genus DiinorjilKidon. 



The teeth of the Ichtliyosaur have a simple more or less 

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



' Biicklantl, Bridgcwatcr Treatise, vol. i. p. 237. 



