894 



TEETH. 



The microscopical examination of the struc- 

 ture of the Iguanodon's teeth thus contributes 

 additional evidence of the perfection of their 

 adaptation to the offices to which their more 

 obvious characters had indicated them to 

 have been destined. 



To preserve a trenchant edge, a partial 

 coating of enamel is applied ; and, that the 

 thick body of the tooth might be worn away 

 in a more regularly oblique plane, the dentine 

 is rendered softer as it recedes from the 

 enameled edge by the simple contrivance of 

 arresting the calcifying process along certain 

 tracts of the inner wall of the tooth. When 

 attrition has at length exhausted the enamel 

 and the tooth is limited to its functions as a 

 grinder, a third substance has been prepared 

 in the ossified remnant q the pulp to add to 

 the efficiency of the dental instrument in its 

 final capacity. And if the following reflections 

 were natural and just after a review of the 

 external characters of the dental organs of 

 the Iguanodon, their truth and beauty be- 

 come still more manifest as our knowledge 

 of their subject becomes more particular and 

 exact : 



" In this curious piece of animal mechanism 

 we find a varied adjustment of all parts and 

 proportions of the tooth, to the exercise of 

 peculiar functions, attended by compensations 

 adapted to shifting conditions of the in- 

 strument, during different stages of its con- 

 sumption. And we must estimate the works 

 of nature by a different standard from that 

 which we apply to the productions of human 

 art, if we can view such examples of me- 

 chanical contrivance, united with so much 

 economy of expenditure, and with such anti- 

 cipated adaptations to varying conditions in 

 their application, without feeling a profound 

 conviction that all this adjustment has re- 

 sulted from design and high intelligence."* 



Varanians. In the great Crocodilian Moni- 

 tor (Varanus crocndilinus), the large fixed com- 

 pressed teeth, of which there may be about 

 seven in each upper maxillary bone and six in 

 each premandibular, are anchylosed by the 

 whole of their base and by an oblique surface 

 leading upwards on the outer side of the 

 tooth to a slight depression on the oblique 

 alveolar surface, as in the Var. striatus. 

 The base of the tooth is finely striated, the 

 lines being produced by inflected folds of the 

 external cement, as in the Ichthyosaur and 

 Labyrinthodon, but they are short and straight, 

 as in those of the former genus. The alveolar 

 channel or groove has scarcely any depth ; 

 but the anchylosed base of the tooth is applied 

 to an oblique surface, terminating in a sharp 

 edge, from which the outer side of the free 

 crown of the tooth is directly continued. 

 The great farauus, like the variegated species 

 manifests its affinity to the Crocodilians in 

 the number of successive teeth which are in 

 progress of growth to replace each other ; but 

 from the position in which the germs of the 

 successional teeth are developed, the more 



* Buckland's Briclgewater Treatise, vol. i. p. 249. 



advanced teeth in this species, as in the Var. 

 varicgatus, do not exhibit the excavations that 

 characterise the same parts of the teeth of 

 the Enaliosaurs and Crocodiles. 



Thecodonts. We have seen that among the 

 inferior or squamate Saurians there are two 

 leading modifications in the mode of attach- 

 ment of the teeth, the base of which may be 

 either anchylosed to the summit of an alveolar 

 ridge, or to the bottom of an alveolar groove, 

 and supported by its lateral wall. These 

 modifications are indicated respectively by 

 the terms " Acrodont " and " Pleurodont." 

 A third mode of fixation is presented by some 

 extinct Saurians, which, in other parts of 

 their organisation, adhere to the squamate or 

 Lacertine division of the order, the teeth 

 being implanted in sockets, either loosely or 

 confluent with the bony walls of the cavity ; 

 these I have termed the " Thecodont " * La- 

 certians : the most ancient of all Saurians 

 belong to this group ; viz. the Thuringian 

 Monitor, or Protorosaurus, and the Palceo- 

 saunts of the dolomitic conglomerates near 

 Bristol. The compressed Varanian form of 

 tooth, with trenchant and finely dentated 

 margins, which characterised the ancient Pa- 

 loeosaur and Chadeiodon, is continued in the 

 comparatively more recent and gigantic species 

 of terrestrial lizard, of which the remains were 

 discovered by Dr. Buckland in the oolite of 

 Stonesfield, by whom the peculiarities of the 

 jaws and teeth have been accurately and gra- 

 phically described in the following words : 



" From these remains we learn that the 

 animal was a reptile, closely allied to some of 

 our modern lizards ; and viewing the teeth as 

 instruments for providing food to a carni- 

 vorous creature of enormous magnitude, they 

 appear to have been admirably adapted to the 

 destructive office for which they have been 

 designed. Their form and mechanism will be 

 best explained by reference to the figures. 



" The outer margin of the jaw rises nearly 

 an inch above its inner margin, forming a 

 continuous lateral parapet to support the 

 teeth on the exterior side, where the greatest 

 support was necessary, whilst the inner margin 

 throws up a series of triangular plates of 

 bone forming a zigzag buttress along the 

 interior of the alveoli. From the centre of 

 each triangular plate, a bony partition crosses 

 to the outer parapet, thus completing the 

 successive alveoli. The new teeth are seen 

 in the angle between each triangular plate, 

 rising in reserve to supply the loss of older 

 teeth, as often as progressive growth, or ac- 

 cidental fracture, may render such renewal 

 necessary, and thus affording an exuberant 

 provision for a rapid succession and resto- 

 ration of these most essential implements. 

 They were formed in distinct cavities, by the 

 side of the old teeth, towards the interior 

 surface of the jaw, and probably expelled 

 them by the usual process of pressure and 

 absorption, insinuating themselves into the 

 cavities thus left vacant. This contrivance 



* Odontography, part ii. p. 2GG. 



