TEETH. 



893 



for the renewal of teeth is strictly analogous 

 to that which takes place in the dentition of 

 many species of existing lizards. 



" In the structure of these teeth we find a 

 combination of mechanical contrivances ana- 

 logous to those which are adopted in the 

 construction of the knife, the sabre, and the 

 saw. When first protruded above the gum, 

 the apex of each tooth presented a double 

 cutting edge of serrated enamel. In this stage, 

 its position and line of action were nearly 

 vertical, and its form, like that of the two- 

 edged point of a sabre, cutting equally on 

 each side. As the tooth advanced in growth 

 it became curved backwards in the form of a 

 priming-knife, and the edge of serrated enamel 

 was continued downwards 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 in- 

 creased by the expansion of its side. Had 

 the serrature continued along the whole of 

 the blunt and convex portion of the tooth, it 

 would in this position have possessed no 

 useful cutting power ; it ceased precisely at 

 the point beyond which it could no longer be 

 effective. In a tooth thus formed for cutting 

 along 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 contrivances 

 which human ingenuity has also adopted in 

 the preparation of various instruments of 

 art." * 



The teeth of the Megctlosaur 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, and when slightly magnified 

 are seen to be rounded, and separated by 

 slight basal grooves ; the smooth and polished 

 enamel upon the sides of the crown presents 

 a finely wrinkled appearance; the remains of 

 the pulp are converted into a coarse bone in 

 the completely formed tooth. 



Enaliosaurs. The teeth of the Ichthyo- 

 sauri have a simple, more or less acutely 

 conical form, with a long and, usually, ex- 

 panded or ventricose base, or implanted fang. 

 They are confined to the intermaxillary, max- 

 illary, and pr eman dibular bones, in which 

 they are arranged in a pretty close and un- 

 interrupted series, and are of nearly equal 

 size. They consist of a body of unvascnlar 

 dentine, invested at the base by a thick layer 

 of cement, and at the crown by a layer of 

 enamel, which is itself covered by a very thin 

 coat of cement ; the pulp-cavity is more or 



* Bridgewatcr Treatise, vol. i. p. 237. 



less occupied in fully-formed teeth by a coarse 

 bone. The external surface of the tooth is 

 marked by the longitudinal impressions and 

 ridges, but the teeth vary both as to outward 

 sculpturing and general form in the different 

 species.* 



The chief peculiarity of the dental system 

 of the Ichthyosaur is the mode of the im- 

 plantation of the teeth ; instead of being an- 

 chylosed to the bottom and side of a con- 

 tinuous shallow groove, as in most Lacertians, 

 or implanted in distinct sockets, as in the The- 

 codon, Megalosaur, or Pterodactyle, they are 

 lodged loosely in a long and deep continuous 

 furrow, and retained by slight ridges between 

 the teeth, along the sides and bottom of the 

 furrow, and by the gum and organised mem- 

 branes continued into the groove and upon 

 the base of the teeth. 



The germs of the new teeth are developed 

 at the inner side of the base of the old ones. 



Crocoddia. The best and most readily 

 recognisable characters by which the existing 

 Crocodilians are grouped in appropriate genera, 

 are derived from modifications of the dental 

 system. 



In the Caimans (genus A/ligator) the teeth 



, 1818 ^ 2-222 . 

 vary in number from rx"iQ to - ? : the 



fourth tooth of the lower jaw, or canine, is 

 received into a cavity of the palatal surface of 

 the upper jaw, where it is concealed when 

 the mouth is shut. In old individuals the 

 upper jaw is perforated by these large in- 

 ferior canines, and the fossae are converted 

 into foramina. 



In the Crocodiles (genus Crocodilus) the 

 first tooth in the lower jaw perforates the 

 palatal process of the premaxillary bone 

 when the mouth is closed ; the fourth tooth 

 in the lower jaw is received into a notch exca- 

 vated in the side of the alveolar border of 

 the upper jaw, and is visible externally when 

 the mouth is closed. 



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. 



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 as contrasted with the 

 blunt mammillate summits of the posterior 

 teeth in the alligator (fig. 573.). The teeth 

 of the (Javial are subequal, most of them pre- 

 sent the form of crown, shown in fig. 572., 

 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. 



* Odontograpliy, pi. 73. 



