COMPLEX AND COMPOUND TEETH. 287 



when it 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 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, bnt is 

 greater at the base. All the inflected folds of cement, at 

 the base of the tooth, have 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 pene- 

 trate to a less distance into the substance of thq tooth. 

 Hence, in such a section as is delineated (Fig. 56), it will 

 be observed that some of the convoluted folds, as those 

 marked cc^ extend near to the centre of the tooth ; others, as 

 those marked c', reach only about half-way to the centre ; 

 and those folds, c", which, to use a geological expression, 

 are " cropping out," penetrate to a very short distance into 

 the dentine, and resemble, in their extent and simplicity, 

 the converging folds of cement in the fangs of the tooth 

 of the ichthyosaurus. 



The disposition of the dentine, cZ, is still more compli- 

 cated than that of the cement. It consists of a slender, 

 central, conical column, excavated, by a conical pulp- 

 cavity, «, for a certain distance from the base of the tooth ; 

 and this column sends from its circumference, radiating 

 outwards, a series of vertical plates, which divide into 

 two, once or twice, before they terminate at the periphery 

 of the tooth. Each of these diverging and dichotomizing 

 plates gives off, throughout its course, smaller processes, 

 which stand at right angles, or nearly so, to the main 

 plate. They are generally opposite, but sometimes alter- 

 nate ; many of the secondary plates or processes, which are 



