ROOM III. STRUCTURE OF TOOTH OF THE IGUANODON. 239 



present a broken or chipped appearance, but not a smooth, 

 flat-worn surface as in the Iguanodon. 



INTIMATE STRUCTURE OF THE TEETH OF THE IGUANODON. 

 Mr. Tomes, F.R.S., whose original and profound microscopic 

 investigations have shed important light on the intimate 

 structure of osseous and dental tissues, has favoured me with 

 the following notes on the organization of the teeth of the 

 Iguanodon. 



" The teeth of the Iguanodon present structural peculiarities which, 

 with our present experience in dental tissues, can be confounded with 

 those of no other animal. The enamel is reptilian in character, that is, 

 it exists as a thin layer, not exceeding the 200th of an inch at the thickest 

 part, and in many places is even thinner; and then it has the usual 

 structureless appearance, with faint wavy markings, in contour lines with 

 the surface of the dentine. Here and there, however, faint lines may be 

 seen proceeding from the surface of the dentine to that of the enamel, 

 which, together with the disposition to break in the direction of the 

 lines, indicates pretty surely the existence of fibres. 



" The dentine of the tooth of the Iguanodon is very remarkable when 

 considered in connexion with the position of the animal in the scale of 

 vertebrata. The dentinal tubes are well marked. They make a bold 

 double curve in their passage from the pulp-cavity to the surface, in 

 addition to the minute undulations which characterize them in every 

 part of their course, and in no part are they free from short, minute, 

 ragged, hair-like branches, which in a thick section give a confused 

 appearance to the tissue. In a longitudinal section of a tooth, the tubes 

 have a diameter varying from the 10,000th to the 15,000th of an inch, 

 which is preserved to near their termination at the surface of the enamel, 

 into which tissue a few are continued a perceptible distance. The pulp- 

 cavity is marked by a series of indentations at tolerably regular inter- 

 vals. From the recesses of these, vascular canals proceed into the 

 substance of the dentine, and follow the course of the dentinal tubes, till 

 near the periphery of the tooth, when they terminate in dilated extre- 

 mities, or turn and follow a parallel course till they regain the pulp 

 cavity. As the tooth becomes worn, these canals are filled by a trans- 

 parent, almost structureless tissue, in the manner described in my paper 

 on the teeth of rodents. 1 It should also be remarked that the dentinal 

 tubes are connected with them through their branches only, and by 

 these but sparingly. Professor Owen, in his account of the tooth of the 

 Iguanodon, (' Odontography,' p. 251,) compares the vascular or medul- 

 lary canals to those which occur in the inner dentine of the tooth of the 

 Megatherium. In the latter, however, the canals are far more numerous 

 than in the Iguanodon, and, moreover, the dentine in which they occur 

 is dissimilar. In it the dentinal tubes are so much interfered with by 

 the very numerous vascular canals that they become irregular, and 

 indeed can scarcely be called tubes; they are irregular cells, minute, and 



1 "Philos. Trans." 1850. 



