LABYRINTHODONTS. 215 



verse diameter of one of the posterior of these serial teeth, where it 

 emerges from the socket, is three lines ; the same diameter of the 

 anterior serial tooth is one line and a half; its length, four lines and a 

 half. The great laniary teeth {a) appear to be three in number in each 

 symphysis, and the one nearest the symphysis is somewhat larger 

 than the others, but they are probably not all in place and use at the 

 same time. The greatest diameter of the sub-compressed base of the 

 largest of these fangs is five lines ; its length, judging from an entire 

 tooth of the same species, must have been at least one inch and a 

 half. The lines of the inflected cement form well marked longitu- 

 dinal stricc around the basal half of the tooth, and the interspaces 

 of the striae form convex ridges, as in the large tooth, the labyrinthic 

 structure of which was described at p. 201. These ridges are 

 fewest near the termination of the strise, being divided and multiplied 

 by new longitudinal striae, caused by new inflected folds of the 

 cement near the basis of the tooth ; the apical half of the tooth has 

 a smooth and polished external surface ; the pulp-cavity is con- 

 tinued of small size into the centre of this part of the tooth. In the 

 serial teeth, which, except their less gradual diminution of size, cor- 

 respond with the anterior larger tusks, the central pulp -cavity is more 

 quickly obliterated ; the texture of the teeth is dense and brittle. 



I have examined the structure of the serial teeth in a small 

 fragment of the upper jaw of the Labyrinthodon pachygnathus, 

 from the same locality as the lower jaw ; this fragment was 

 three inches and a half in length, and included twenty-four sockets of 

 the serial teeth, the alternate teeth being in place though broken. 

 These teeth precisely corresponded in size and form with those of the 

 lower jaw. The labyrinthic structure is confined to the basal half of 

 the tooth, where it is indicated by the external striation, and becomes 

 more complex as it approaches the base. The blending of the exter- 

 nal layer of cement with the dentine, as exhibited by a transverse sec- 

 tion of the tooth above this part, well illustrates the principle of the 

 more complicated modifications of the labyrinthic structure first dis- 

 covered. The processes of dentine which radiate from the pulp-cavity 

 (PI. 63 B, fig. 1, ft), are twelve in number at the line of the section 

 here described, but most of them divide in their course outwards ; 

 the corresponding pulp-fissures diverge in straight lines, bifurcate 



