LABYRINTHODONTS. 203 



fourths of the tooth, by decreasing in number as the tooth diminishes 

 in thickness ; they finally altogether disappear about half an inch from 

 the summit of the tooth : and at this part, from the analogy of other 

 species of Labyrinthodon, it is to be presumed that the structure of the 

 tooth of the Lah. Jaegeri may be more simple. 



The dentine or body of the tooth is invested by only a very thin 

 layer of cement, and it is a vertical fold or duplicature of this cement 

 which penetrates the substance of the tooth at each of the strise, 

 which, as before observed, are arranged at intervals of about one line, 

 around the whole circumference of the tooth. The inflected fold runs 

 straight for about half a line, and then becomes wavy, the folds rapidly 

 increasing in breadth as they recede from the periphery of the tooth ; 

 the first two, three, or four undulations are simple ; then their con- 

 tour itself becomes broken by smaller or secondary undulations ; 

 these become stronger as the fold approaches the centre of the 

 tooth, when it shghtly increases in thickness, and finally terminates 

 by a slight dilatation or loop close to the pulp-cavity, from which the 

 free margin of the inflected fold of cement is separated by an ex- 

 tremely thin layer of dentine. The number of the inflected converging 

 folds of dentine is about fifty at the middle of the crown of the tooth, 

 but it must be greater at tlie base. All the inflected folds of cement 

 at the base of the tooth have probably the same complicated disposi- 

 tion with increased extent ; but, as they approach their termination 

 towards the upper part of the tooth, they also gradually diminish in 

 breadth, and consequently penetrate to a less distance into the sub- 

 stance of the tooth. Hence, in such a section as is deUneated, it will be 

 observed that some of the convoluted folds, as those marked c c PI. 64 a, 

 fig 1, extend near to the centre of the tooth ; others, as those marked 

 d d, reach only about half way to the centre ; and those folds, which, 

 to use a geological expression, are ' cropping out', penetrate to a very 

 short distance into the dentine, and resemble in their extent and 

 simpUcity the converging folds of cement in the fangs of the tooth of 

 the Ichthyosaurus. 



The disposition of the dentine in the tooth of the Labyrinthodon 

 Jaegeri is still more complicated than that of the cement. It consists 

 of a slender, central, conical column, excavated by a conical pulp-cavity 



