266 THECODONTOSAURUS. 



replace each other : but from the position in which the germs of the 

 successional teeth are developed, the more advanced teeth in this 

 species, as in the Var. variegatus, do not exhibit the excavation that 

 characterize the same parts of the teeth of the Enaliosaurs and 

 Crocodiles. 



THECODONTS. 



110. We have already seen that among the inferior or squamate 

 Saurians there are two leading modifications in the mode of attach- 

 ment of the teeth, the base of which may be either anchylosed to the 

 summit of an alveolar ridge, or to the bottom of an alveolar groove and 

 supported by its lateral wall : these modifications were indicated by 

 the terms * acrodont ' and * pleurodont.' A third mode of fixation is 

 presented by some extinct Saurians, which, in other parts of their 

 organization, adhere to the squamate or Lacertine division of the 

 order; the teeth being implanted in sockets, either loosely, or confluent 

 with the bony walls of the cavity : these may be termed the ' theco- 

 dont '(1) Lacertians ; the most ancient of all Saurians belong to this 

 group. 



111. Thecodontosaurus. — In the dolomitic conglomerate at Red- 

 land near Bristol, a formation considered to belong to the oldest or 

 lowest division of the new-red-sandstone series, remains of reptiles have 

 been discovered by Dr. Riley and Mr. Stutchbury(2) which are allied, 

 in the form of their teeth to the typical Varanian monitors, but dif- 

 fer in having the teeth imbedded in distinct sockets : to this condi- 

 tion, however, the Varani, among the squamate saurians, make an 

 approach, in the shallow cavities containing the base of the teeth 

 along the bottom of the alveolar groove. 



In the ancient extinct genus in question the sockets are deeper, 

 and the inner alveolar wall is nearly as high as the outer one : the 

 teeth are arranged in a close-set series, slightly decreasing in size 

 towards the posterior part of the jaw ; each ramus of the lower jaw is 

 supposed to have contained twenty-one teeth. These are conical, 



(1) OT)Kt], a sheath j oSsc a tooth. 

 (.2) Geol. Transactions, 1836, p. 349. 



