360 Transactions. — Geology. 



passage from the centre to the periphery of the teeth, xinqnestiohably 

 establish the view that the substance of the tooth is not built up by successive 

 deposition of layer after layer of bony corpuscles, as maintained by Cuvier 

 and the " excretion " theorists, but that the growth of the tooth is carried on 

 by spreading tubuli, through which the nutrition is preserved and necrosis 

 and absorption eifFected. 



It is not the teeth only which afford interesting links in the chain of 

 vertebrate animals. The structure of the jaw-bone is remarkable in the 

 Lacertse. It is built up of three or four bones, adjusted and anchylosed 

 together ; and it has been somewhat fancifully supposed by Buckland and 

 Owen that this arrangement gives, in the case of the Crocodile and the 

 Ichthyosaurus, additional strength to the jaw, and renders it better able to 

 resist the violent concussion of their formidable mandibles when snapping at 

 their prey. 



In the mandibular bone of the Leiodon, it will be seen from PI. XXV. 

 that it is made up of four bones spliced together, viz., the dentary, the 

 coronoid, the angular, and the splenial or opercular. The section from which 

 the drawing is taken at once reveals to us that it formed part of the fossil 

 remains of a Lacertian. 



In the Lacertse the jaw-bone, in most cases, presents only a sort of pai-apet 

 on the outer side, and the teeth are fixed to it by a bony mass occupying the 

 place of their root, and incorporated organically both with the tooth and the 

 jaw-bone. In the Mosasaurus and the Ichtliyosauri there is an inner parapet 

 as well. In the Leiodon there is both an inner and outer parapet, with a deep 

 fossa between, as seen in PI. XXY. 



The crown of the tooth in the Leiodon has a simple conical form. It is 

 polished, striated, and of a dark colour. The numerous tine longitudinal 

 strise sharply marked on the polished surface of the tooth are owing to the 

 splitting down of the crusta petrosa. The slits are well shown in PI. XXIV. 

 They do not extend to the dentine beneath. The margins, separating the 

 outer from the inner, are well defined, and are sometimes broken by slight 

 elevations, but this irregularity in no case gives a dentated character to the 

 outline. PL XXIV. B shows the outline of a transverse section of the middle 

 of the crown of a tooth. 



The base, or fang, of the tooth presents a tapering subventricose form, and 

 is implanted, as in the Mosasaur, in a cementing alveolus, i-aised in a rounded 

 form from the deep longitudinal fossa formed by the ridges or pai-apets of 

 the dentary bone (PI. XXV.). The teeth, although hollow, do not, like those 

 of the crocodile, contain in themselves the replacing teeth. Owen describes 

 the teeth of the Leiodon as supported on a hillock of bone resting upon the 

 broad alveolar surface of the jaw ; but, in fact, the hillocks, or cementing 



