Knight. — 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 PI. XXY. 



The Leiodon belongs neither to the Pleurodonts, in which the teeth are 

 attached to the inner side of the dentaiy bone, or, according to Owen, to an 

 exterior alveolar plate of bone, the inner plate not being developed ; nor 

 to Pleodonts or to Cselodonts 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 (PI. XXY.). 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 succesoional, 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 alveohis, and, pressing against the alveolus adjoining the base of 

 the pxiraitive 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 XXIY. A, that the 

 gei-m 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 

 it would seem as if the young tooth was growing in a vascular, or, at any rate, 

 not a fully ossified pulp, wliich, in the fossil state, has become replaced by a 

 homogeneous deposit extfsndiug up the pulp-cavity of the adjoining primitive 

 tooth. Tlie germ toucb'-'S the dentaiy 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 p-oti'uded tooth. 



In order to obtain satisfactory views of the structure of the teeth, vei-tical 

 and transverse sections were made. The transverse section, PL XXIY.F, shows, 

 besides the dentine, an outer layer, the crusta fetrosa^ of extreme tenuity, 

 being less than -03 inch thick. The divisional lines, forming block-like masses, 

 are seen on the sui-fuce of the tooth as longitudinal striae, as mentioned above. 

 The dentine consists of a single pulp-cavity and a system of excessively fine 

 calcygerous tubuli at regular and minute intervals from each other, radiating 

 from the pulp-cavity at right angles to the peri]3hery of the tooth. The walls 

 of the tubuli are diaphanous, and the interspaces occupied by a bony or 

 calcified deposit, giving a somewliat cellular a]:)i)earance to the dentine (E), 

 with occasionally a pinnate arrangement of short branches sliooting from the 

 tubuli nearly across the intervening spaces. Owen notices a similai- a}>])earance 

 in the tooth of the Mastodon. 



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