254 LACERTIANS. 



usual characters of the texture of the bone in the higher reptiles. The 

 dentine differs, like that of existing Lacertians, from the dentine of the 

 Iguanodon in the entire absence of the numerous medullary canals 

 which form so striking a characteristic of the more gigantic Wealden 

 reptile. The main calcigerous tubes are characterized by the slight 

 degree of their primary inflections ; they are continued in an unusually 

 direct course from the pulp-cavity to the outer surface of the dentine, 

 at nearly right angles with that surface, but slightly inclined towards 

 the expanded summit of the tooth. They are chiefly remarkable for 

 the large relative size of their secondary branches, which diverge from 

 the trunks in irregular and broken curves, the concavity being always 

 turned towards the pulp-cavity. In most parts of the tooth, the 

 number of these branches obscures even the thinnest sections. 



The ossified pulp exhibits the parallel concentric layers of 

 the ossified matter surrounding slender medullary canals and inter- 

 spersed with irregular elliptical radiated cells. 



LACEKTIANS. 



105. The genera of the typical family of the squamate Saurians 

 are arranged in two subfamiUes by MM, Dumeril and Bibron, the 

 chief characteristics of which are derived from the dental system. 



In the first group, the teeth are solid, or without any internal 

 cavity, and are very firmly anchylosed by their base to the alveolar 

 groove upon the inner side of the jaw ; so that the extremity of the 

 tooth is slightly directed outwards : the species which present this 

 character are called ' pleodont' Lacertians. (1) 



In the second group, the teeth are excavated by a sort of canal, 

 and are less firmly fixed to the jaws, being applied vertically, like 

 piles or buttresses, against the outer alveolar parapet, but not adhering 

 by their base : this group is called ' Coelodonts.'(2) The group of 

 Pleodonts includes the genera Crocodilurus, Thorictes, Neusticurus, 

 Aporomera, Monitor^ Ameiva, Cnemidophora, Dicrodon, Acrantus and 

 Centropyx. 



The Coelodont Lacertians comprise the genera Tachydromus^ 



(1) nXeoS, full. (2) icoiXog, hollow. 





