44 



THE CERATOPSIA. 



middle of the jaw, while the number of teeth in each longitudinal series may be upward of 

 forty, varying in number according to the position and perhaps also according to the age of 

 the individual. In a single transverse section through the jaw there is never more than one 

 vertical series of teeth in the Ceratopsia. Thus the number of teeth seen in a transverse section 

 of a jaw at any point will equal the number of longitudinal series of teeth present in that part 

 of the jaw. 



REPLACEMENT OF THE TEETH. 



The method of replacement of the teeth in the Ceratopsia is somewhat peculiar. If trans- 

 verse sections be made through the jaw at the bottom of each vertical series of teeth an incipient 



£ 



Fig. 43. — A. Back view of single vertical series ol inferior teeth of Triceratops flabellaius, showing arrangement in jaw and method of replace- 

 ment; B, external view of same; C, internal view of same, a. Worn surface of functional tooth; b, incipient tooth. Natural size. 



tooth will be seen with the crown fairly well developed, as shown at h, fig. 43, with thin walls 

 and a pulp cavity extending quite to the apex, while below the crown there is a single root, 

 widely expanded and with very thin walls, as shown in the cross section at A, fig. 45. The 

 crown of this basal tooth stands nearly vertical and it is inserted into the pulp cavity at the 

 base of the tooth immediately above. The anterior and posterior walls of the pulp cavity in 

 this second tooth have been much constricted, however, by the adjacent anterior and posterior 

 basal angles of the adjacent teeth of the immediately preceding and succeeding vertical 

 series of teeth, as shown at B, fig. 45. The crown of this second tooth in the series instead 

 of occupying a vertical position in the jaw, as does that of the basal tooth, is inclined inward if 

 an upper tooth and outward if a lower. Its apex is inserted into the pulp cavity of the tooth 



