202 LABYRINTHODONTS. 



process of the diverging layers of the dentine, and thus is produced 

 the singularly complicated appearance which a transverse section of 

 the tooth of the Labyrinthodon or Mastodonsaurus exhibits. 



The external longitudinal flutings of the base of the tooth of the 

 Ichthvosaur are much coarser, and more indicative of the converg- 

 ing vertical folds of the cement, than are the corresponding longi- 

 tudinal lines on the exterior of the tooth of the Labyrinthodon ; 

 which is owing to the layer of the inflected cement being much 

 thicker in the Ichthyosaur. The external striae of the Labyrintho- 

 don's tooth are too little conspicuous to attract particular attention, 

 or to indicate that they are the Unes of inflection of a series of 

 extensive vertical folds of the external substance. Accordingly Professor 

 Jaeger describes the tooth of the Mastodonsaurus as being longi- 

 tudinally striated on the superficies, noting where the strice termi- 

 nate their relative distances, and where they are most marked : the 

 texture of the tooth, where it was exposed by fracture, he pronounces, 

 as indeed it appears to the naked eye, to be uniform, homogeneous 

 and compact(l), and he concludes his description by stating that the 

 tooth resembles most closely that of the Lacerta nilotica and of some 

 species of Monitor. 



The teeth, however, of those species of Varanus, Lacerta, Monitor, 

 and other Saurian genera which I have submitted to microscopic 

 investigation, have all presented the usual structure of simple Saurian 

 teeth. 



The portion of the tooth of the Labyrinthodon Jaegeri from 

 which the sections here described, were prepared, included 

 about the middle third part of a tusk nearly as large as the one 

 figured by Prof. Jaeger (2) . That tooth is three inches and a 

 half in length, and one inch and a half in breadth at the base, 

 whence it gradually contracts, with a slight bend, towards the 

 apex: this is obtuse, with a slightly depressed summit ; it is three lines 

 in diameter, and presents a small rising in the centre of the terminal 

 depression. The external longitudinal striae are regularly arranged 

 with intervals of about a fine at the base of the tooth ; and they main- 

 tain nearly the same relative position throughout the lower three 



(1) Jaeger, loc. cit. p. 3G. (2) Loc. cit, tab. iv. fig. 4. 



