of Wirtemberg and Wanvickshire. 509 



observed, are arranged at intervals of about one line around the whole circum- 

 ference of the tooth. The inflected fold runs straight towards the centre of the tooth 

 for about half a line, and then becomes wavy, the undulations rapidly increasing in 

 breadth ; the first two, three, or four undulations are simple, then their contour 

 itself becomes broken by smaller or secondary undulations, and these become more 

 numerous as the fold approaches the centre of the tooth, near which it shghtly 

 increases in thickness, and finally terminates by a dilated loop or extremity (as seen 

 in the transverse section) close to the pulp-cavity, from which the free margin of 

 the inflected fold of cement is separated by an extremely 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 the base. 



All the inflected folds of cement at the base of the tooth have probably the dis- 

 position and extent above described, but as they approach their termination towards 

 the upper part of the tooth (and we have seen that they progressively decrease in 

 number as the tooth diminishes in size) they also gradually diminish in breadth, 

 and consequently penetrate a less distance into the substance of the tooth. 



Hence, in such a section as I have delineated, we observe that some of the 

 convoluted folds, or those marked b, b (fig. 1, p. 506.), extend to near the centre of 

 the tooth. Others reach only about half-way to the centre ; and those folds (&', &') , 

 which, to use a geological expression, are " thinning out," penetrate to a very short 

 distance into the dentine, and resemble in their extent and simplicity the con- 

 verging folds of cement in the fang of the tooth of the Ichthyosaurus. 



The disposition of the dentine in the tooth of the Labyrinthodon is still more 

 complicated than that of the csementum. The dentine, or the main constituent of 

 the tooth, consists of a slender central conical column, or ' modiolus,' hollow for a 

 certain distance from its base, and radiating outwards from its circumference a 

 series of vertical plates, which divide into two, or dichotomize once or twice before 

 they terminate at the periphery of the tooth. 



Each of these diverging and dichotomizing vertical plates gives off" throughout 

 its course narrower vertical plates, which stand at right angles or nearly so to the 

 main plate ; they are generally opposite, but sometimes alternate. Many of the 

 secondary plates which are given off" near the centre of the tooth also divide into 

 two before they terminate : their contour is seen in the transverse section to partake 

 of all the undulations which have been described as characterizing the inflected folds 

 of cement which invest the dentinal lamellse and separate them from each other. 



The central pulp-cavity (Cut 1 , a, p. 506.) is reduced to a mere line about the up- 

 per third of the tooth, but fissures radiate from it corresponding in number with the 

 radiating plates of the dentine. One of these fissures is continued along the middle 

 of each plate, dividing where this divides, and extending along the middle of each 



VOL. VI. SECOND SERIES. 3 U 



