240 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



without definite arrangement. In the Iguanodon, on the contrary, 

 the dentinal tubes and their branches are well marked, and definitely 

 arranged. 



" On careful comparison it will be found that the dentine of the teeth 

 of this great vegetable-feeding reptile pretty closely resembles that of 

 many of the larger herbivorous mammalia, and more especially the 

 ruminants. In these we find the dentine traversed, though less abun- 

 dantly, by similar vascular canals. In the American Tapir, and also in 

 the Solipedes, a similar condition is observable. In mammalian dentine 

 the parietes of the dentinal tubes are well marked, especially in the 

 ruminants, where they are extremely thick ; this cannot be said to be 

 the case in the Iguanodon, for although the parietes may be seen in 

 a favourable section, yet they are by no means so distinct or so thick ; 

 neither, indeed, are the dentinal tubes themselves so large as in the great 

 mammalian herbivora. So far as my own experience goes, the presence 

 of vascular canals in the substance of the dentine as a constant character, 

 is confined to the teeth of the vegetable feeders. 



"Professor Owen has described, (' Odontography,' p. 252,) a third 

 substance in the tooth of the Iguanodon. He says, ' The remains of the 

 pulp in the contracted cavity of the completely formed tooth, are con- 

 verted into a dense but true osseous substance, characterized by minute 

 elliptical radiated cells, whose long axis is parallel with the plane of the 

 concentric lamellae which surround the few and contracted medullary 

 canals in this substance.' I have seen the concentric lamellae in the 

 situation described by Professor Owen, and these have been perforated 

 by straggling, irregular, dentinal tubes, but I have failed to observe in 

 this or in any other part of the tooth elliptical radiate cells, in other 

 words, bone or cement lacunae. 



" In the central part of these teeth, a dark brown matter is commonly 

 seen. This is for the most part composed of small, oval, ferruginoiis- 

 looking bodies about the size of bone lacunae, which are surrounded by 

 imperfectly formed crystalline matter. This broAvn substance occupies 

 the pulp-cavity, and often extends a short distance into the vascular 

 canals. In a thick section it might on a casual inspection be taken for 

 cementum, but a more careful observation would at once show it to be 

 a mere product of fossilization. 



" If I had to describe the tooth of the Iguanodon from its tissues in 

 a few words, I should say it was a tooth having herbivorous (mam- 

 malian ?) dentine, with reptilian enamel." 



LOWER JAW OF THE IGUANODON. Ligns. 54 and 55. The 

 importance of discovering the peculiar construction of the 

 maxillary organs which had impressed such anomalous cha- 

 racters on the teeth of a reptile, as to impart to those 

 instruments so striking a resemblance to the incisors of herbi- 

 vorous mammalia as to mislead the most eminent anatomist 

 of modern times, could not be estimated toojiighly ; and for 

 many years, my curiosity and interest were painfully excited 

 by the desire of solving the mystery in which the subject 



