360 Transactions.—(Geology. 
passage from the centre to. the periphery of the teeth, unquestionably 
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 effected. 
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 
Lacerte. 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 Zeiodon, it will be seen from РІ. 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 Lacertz the jaw-bone, in most cases, presents only a sort of parapet 
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 Ichthyosauri 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 Pl. XXV. 
The crown of the tooth in the Leiodon has a simple conical form. It is 
polished, striated, and of a dark colour. The numerous fine longitudinal 
strie 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 Pl. 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 Mosasawr, in a cementing alveolus, raised in a rounded 
form from the deep longitudinal fossa formed by the ridges or parapets of 
the dentary bone (Pl. 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 
