Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/281

 dentine, coated with a thin layer of enamel. The dentinal tubules pass straight from the pulp-cavity towards the enamel without waving, in which respect, and in the almost complete absence of the concentric granular rings, which are so conspicuous a feature in the dentine of many crocodilian teeth, the dentine of these, toeth resembles that of the unfluted apex of anlchthyosaurian tooth-crown. The enamel, however, is thinner than that of several sections of Liassic Ichthyosaurian teeth of about the same diameter. At the base of the fang the dentine, owing to the presence of a large pulp- cavity, becomes thin ; and its contour is here slightly aud irregu- larly indented, but the indentations are quite insignificant in com- parison with the deep inflexions which distinguish the tooth-fang in Ichthyosaurus. The pulp-cavity extends into the base of the crown. Its apex is filled with spar ; and its lower end contains a plug of ossified tooth-pulp, continuous through the open end of the cavity with the external cementum, its vascular canals freely inosculating with those of the cementum in this situation. The vascular canals of the cementum are large and numerous. Their principal branches take the direction of the long axis of the fang, running from the base toAvards the neck. The bony tissue channelled by them abounds in lacunas, which do not differ in any essential particular from those of the bones which bear the teeth.

The connexion of the teeth with the jaw appears to resemble that which obtains in Ichthyosaurus : they are neither soldered to a flange nor implanted in sockets, but they are arranged in a line in an open groove in the upper surface of the jaw. Several of them are dis- placed out of the groove, which shows the absence of anchylosis to the dentary bone, and that they were attached to it only through the medium of the soft tissues. Of the teeth remaining in situ, six occupy 1*5 inch.

The swelling caused by the great development of the cementum gives these teeth some resemblance, but only of a superficial kind, to those of the Chalk " Fossil Fish or Reptile " lately pur- chased by the British Museum, formerly in the collection of Mr. Toulmin Smith, who described it in Charlesworth's ' London Geo- logical Journal,' No. 1. Sept. 1846. Mr. Bowerbank afterwards re- ferred to it in a paper on bone corpuscles. The teeth are here in- serted in bony cups ; and Mr. Smith expressed the opinion that these were shed together with the teeth. The occurrence of gaps in the dentary series from which both cup and tooth are absent pro- bably led to this view ; but there are also several full-sized empty cups without teeth in them, which demonstrates that the cup and tooth do not in every instance simultaneously disappear, and ren- ders it very unlikely that any hard structural connexion existed between them. I think it more probable that the tooth was gene- rally first shed, or detached by violence, and that after this the cup was removed by the abrasion of its thin unprotected edge, and by absorption.

As well as I can judge, iu the absence of sections through the tooth-cup and bone which bears it, the cups are upgrowths from the