44 FOSSIL REPTILIA OF THE 



he states to have come into his possession from " one of the numerous chalk-pits on 

 the Essex side of the Thames " — the side on which the county of Norfolk lies ; and it 

 appears that the teeth described and figured in my ' Odontography ' are not only 

 specifically identical, but once formed part of the same specimen, with that which he 

 has since figured. This may well be, for in the mass of materials which I had been 

 collecting for six years previous to the publication of my ' Odontography ' I found the 

 drawings, which are engraved in PI. 72, figs. 1 and 2 of that work, marked ' from the 

 chalk of Norfolk,' without any other memorandum, and I feel obliged to Mr. Charles- 

 worth for having publicly supplied in 1846, what my memory in 1840 failed to do, 

 viz., the reference to the individual to whom I had been indebted in 1835 for the 

 loan of the originals of those drawings. 



With regard to the question of the nature and affinities of the Leiodon, the 

 additional evidence which the figures published by Mr. Charlesworth afford, is of 

 value. The teeth in that specimen can only be referred to the genus Mosasaurus, as 

 characterised by Cuvier and Goldfuss, on the supposition that they are ' pterygoid teeth.' 

 But, in an extent of an alveolar tract of seven inches, and including five teeth, that 

 tract is slightly concave lengthwise, instead of being convex: and it wants the 

 horizontal platform extended to the outside of the teeth in place, and supporting the 

 nidus of their successors, wdiich characterises the pterygoid bones. 



In my ' Odontography,' I have briefly noticed one of the most common conditions 

 of fossil teeth, in which the pulp-cavity has not been obliterated by calcification of the 

 pulp itself in the lifetime of the animal. Thus, in the section on the teeth of the 

 Ichthyosaurus, it is described in the following passage. "The remains of the pulp, 

 after the formation of the due quantity of dentine, became converted, as in the 

 pleodont lizards, by a process of coarse ossification, into a reticulate, fibrous, or 

 spongy bone ; but it continued open at the crown after the basal part of the tooth was 

 thus consolidated, as is shown in the longitudinal section, (PI. 73, fig. 8,) wherein a 

 is the pulp-cavity, filled with crystallized spath, b the ossified pulp at the base of the 

 tooth." p. 279. In fig. 2*, Tab. IX, A, is reproduced Mr. Charlesworth's figure of the 

 mass of similar siliceous spath, that, in like manner, filled the uncalcified part of the 

 pulp-cavity of the tooth of the Leiodon anceps. Although I should not have called this 

 " a very unlooked for condition of the interior of the tooth," I concur with the Editor 

 of the ' London Geological Journal ' in his hypothesis of the precipitation of the 

 siliceous matter from a fluid. But, at the same time, I am fully conscious how trans- 

 parent a veil such an hypothesis is to our ignorance as to the precise conditions of the 

 precipitation of such matter in the interior of fossil teeth, in the medullary cavities of 

 fossil bones, and in the closed chambers of many polythalamous shells. The only 

 wonder connected with the fact illustrated in T. IX, A, figs. 2 and 2*, is, that any 

 Geologist should deem it an unlooked for or extraordinary one. 



I have described and figured some small detached crowns of the teeth of the Leiodon, 



