THE SKULL. 



45 



above, the anterior and posterior walls of which have now become entirely obliterated, as 

 shown at C, fig. 45, by the crowding of the crowns of the succeeding tooth of the same vertical 

 series and the adjacent teeth of the immediately anterior and posterior vertical series, and 

 instead of one root there are 



now two roots, completely <5 



separated. The external and 

 stronger branch of the root, 

 if the tooth is an upper, is 

 vertical and incloses the 

 greater portion of what still 

 remains of the pulp cavity. 

 That portion of the pulp cav- 

 ity adjoining the tooth im- 

 mediately beneath is with- 

 out a boundary wall, and the 

 keels of the displacing tooth 

 are inclosed by the lateral 

 walls of the remnants of the 

 pulp cavit}^ still remaining 

 in the two branches of the 

 now completely divided root. 

 The internal branch of the 

 root, if the tooth be a lower, 

 is closely applied to the dor- 

 sal surface of the displacing 

 tooth, the keel of which is 

 fitted into the vestigial pulp 

 cavity of the root. The dor- 

 sal root of each tooth is pro- 

 jected as a strong buttress 



far beyond the base of the crown and forms above at its junction with the crown a broad, 

 shallow cavity, into which the extremity of the corresponding root of the tooth above is 

 fitted. The surface of the dorsal root bears a thick covering resembling cementum. In 

 using the terms external and internal I have had reference to the inferior teeth; in the superior 

 teeth the conditions as described above are entirely reversed, 

 as shown in fig. 46. From the above description and the 

 accompanying figures it will be seen that in the Ceratopsia 



Fig. 44.— Internal view of inferior teeth of type of -Triceratops flabellatus, as revealed by remov- 

 ing internal wall of right dentary at anterior extremity of dental magazine, a, Unworn 

 teeth; b, worn teeth. Natural size. 



Fig. 45. — A, Transverse section through root of incipient tooth of type 

 of Triceratops flabellatus; B, transverse section through root of a 

 slightly older tooth of same; C, transverse section of root after it 

 has become completely bifurcated; D, transverse section through 

 root of fully adult tooth. " 



Fig. 46.— A, Superior tooth of Triceratops, front 

 view; B, inferior tooth of same, same view. 

 Natural size. 



the teeth are replaced from below and that each tooth is carried upward on the crowns of three 

 teeth pertaining to three different vertical series and two different longitudinal series. The 

 median of these three displacing teeth is in the longitudinal series next below that of the other 



