Kyient.—On the Teeth of the Leiodon. 361 
alveoli, rest on the thin floor of the dentary fossa, to whose parapets they are 
anchylosed, as shown in Pl. X X V. 
The Zeiodon belongs neither to the Pleurodonts, in which the teeth are 
attached to the inner side of the dentary bone, or, according to Owen, to an 
exterior alveolar plate of bone, the inner plate not being developed ; nor 
to Pleodonts or to Celodonts of Dumeril and Bribon, the former of which 
have solid teeth, attached by their bases to the groove on the inner side of 
the dentary bone, and the latter have hollow teeth, applied like buttresses 
against the outer plate of the dentary bone. The distinctive character of the 
teeth of the Leiodon is their position on elevated cementing alveoli or hillocks; 
hence the term Acrodonts, applied to this Lacertian and the Mosasaurs. 
The fang of the fossil tooth is sharply defined in the vertical section of the 
jaw-bone (PL XXV.) The sides become thin and evanescent below, and the 
extremity, instead of being pointed, forms the wide opening of the pulp-cavity. 
The expanded base of the germ of the successional tooth is developed from 
the inner side of the dental fossa, in the interspaces of the primitive teeth, and 
from within the cementing alveoli. In its growth the young tooth shoots 
through the alveolus, and, pressing against the alveolus adjoining the base of 
the primitive tooth, interferes with its contour, and at length loosens it, and 
causes both it and its tooth to fall. It will be seen, in Pl. XXIV. A, that the 
germ springs from the floor of the dental fossa, and stands with its broad base 
in a cavity communicating with the pulp-cavity of the primitive tooth ; so that 
dt would seem as if the young tooth was growing in a vascular, or, at any rate, 
not a fully ossified pulp, which, in the fossil state, has become replaced by a 
homogeneous deposit extending up the pulp-cavity of the adjoining primitive 
tooth. The germ touches the dentary bone, and must have sprung from a 
vascular membrane covering the floor of the dental fossa, and lining the pulp- 
cavity of the full-grown protruded tooth. 
In order to obtain satisfactory views of the structure of the teeth, vertical 
and transverse sections were made. The transverse section, Pl. X XIV.F, shows, 
besides the dentine, an outer layer, the crusta petrosa, of extreme tenuity, 
being less than ‘03 inch thick. The divisional lines, forming block-like masses, 
are seen on the surface of the tooth as longitudinal striz, as mentioned above. 
The dentine consists of a single pulp-cavity and a system of excessively fine 
caleygerous tubuli at regular and minute intervals from each other, radiating 
from the pulp-cavity at right angles to the periphery of the tooth. The walls 
of the tubuli are diaphanous, and the interspaces occupied by a bony or 
calcified deposit, giving a somewhat cellular appearance to the dentine (E), 
with occasionally a pinnate arrangement of short branches shooting from the 
tubuli nearly across the intervening spaces. Owen notices a similar appearance 
in the tooth of the Mastodon. 
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