TEETH OP RErTILBS. 401 



dentition, by maintaining the serviceable state of the tnsk by 

 virtue of constant renovation of the .substance of one and the 

 same matrix, according to tlie principle manifested in the long- 

 lived and ever-growing tush:s of the Walrus, and the scalpriform 

 incisors of the Kodentia. 



Naturalists have availed themselves of characteristics of the 

 dental system to arrange the genera of the squamate Laccrtians. 

 In the first group, the teeth are solid, or without any perma- 

 nent internal cavity, and are anchylosed by their base to the 

 alveolar groove. The species which present this character are 

 called ' Pleodonts.' In the second group, the teeth are excavated, 

 or retain the pulp-cavity, 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 grouj) is 

 called ' Coclodonts.' 



The Monitor of South America ( Ttipinanihis teguixiii) is 

 an example of the Pleodont group, in which the premaxillary 

 teeth are ten in number. The maxillary teeth vary from ten 

 to fifteen on each side, and increase in size as they are placed 

 farther back : the hindmost teeth are tricuspid in young indi- 

 viduals, and present the form of simple tubercles in the old 

 Monitors. The mandibular teeth, fifteen to eighteen in number 

 in each ramus, correspond in size and form with those above. 

 In the Coelodont group, the ' Swift lizards ' ( Tachydromus) have 

 the pterygoid bones armed with minute teeth. The teeth on both 

 upper and lower jaws are of larger size, and the hinder ones are 

 tricuspid. The true Lizards {Lacerta) have two kinds of teeth 

 quoad form ; the anterior small, conical, and recurved : the pos- 

 terior larger, and bi- or tri-cuspid. Some species have also 

 pterygoid teeth ; as the common Lacerta agilis. 



In the gigantic fossil Sea-monitor of Maestricht (Mosasaurus) 

 the teeth combine the ' Pleodont ' with the ' Acrodont ' characters. 

 All the teeth are slightly recurved, and their peripheral surface 

 is smooth : the crown is pyramidal, with the outer side nearly 

 plane, or slightly convex, and separated by two sharp ridges from 

 the remaining surface, which forms a half-cone. The larger teeth 

 are implanted upon the premaxillary, maxillary, and premandibular 

 bones ; a series of similarly shaped but much smaller teeth are 

 placed upon the pterygoid bones. 



Most of the smooth-scaled Lizards have small mouths and 

 slender sharp teeth, fitted best for insect food : they are usually 

 confined to the upper and lower jaws ; but the medicinal Scink of 

 ancient pharmacy ( Scincus officinalis) has four or five small obtuse 



VOL. T. D D 



