THE SKULL. 



43 



corresponding processes that great rigidity L obtained in this region, where it is absolutely 

 necessary in order to secure perfect control of the mandible. » 



The relations of the various elements of the lower jaw to one another are well shown in 

 fig. 41 and in Pis. VI, XXVII, and XLI. 



THE TEETH. 



Fig. 41. — External view of left ramus of type of Triceratops prorsus. No. 1822, Yale Museum. 

 c, Coronoid process; pd.predentary; d, dentary; on, angular; or, articular; sa, surangu- 

 lar. One-eighth natural size. After Marsh. 



THE UNWORN TEETH. 



The fully adult teeth before being subjected to wear have acutely pointed, conical crowns, 

 as shown in fig. 42. At the base of the crown the transverse diameter exceeds the antero- 

 posterior, while toward the top 

 these dimensions, are reversed 

 and the antero-posterior diame- 

 ter becomes the longer. In 

 each tooth the anterior and pos- 

 terior edges of the crown are 

 produced so as to form anterior 

 and posterior keels, sharp above 

 but rounded below, and finely 

 serrated along the edges, espe- 

 cially toward the apex. In the 

 superior teeth there is a median 

 vertical keel on the inner sur- 

 face of the crown extending from the apex quite to the base, while the external surface of 

 the crown bears a vertical keel situated somewhat posterior to the median line and thus divid- 

 ing the external surface of the tooth into a smaller posterior and larger anterior portion. 

 These conditions are exactly reversed in the teeth of the lower jaw, where the median vertical 

 keel is on the external surface of the crown, while the internal surface is divided by the ver- 

 tical keel into two unequal 

 areas, the anterior of which 

 is the smaller. As in the 

 superior teeth this vertical 

 keel is sharper and better 

 defined than the median ver- 

 tical keel of the opposite 

 side of the tooth. The teeth 

 are apparently two-rooted, 

 though in reality there is in 

 the young tooth only a single 

 root, which is early divided 

 into two branches by the pe- 

 culiar manner in which these 

 teeth replace one another, which will be fully described and illustrated later. In the young 

 tooth the root is single and triangular in cross section. The pulp cavity is large and the walls 

 are extremely thin, especially the anterior and posterior walls of the root just below the crown. 

 In the young tooth the pulp cavity extends well up into the crown, but as the tooth matures 

 the walls thicken and the cavity is confined to the root. 



Tig. 42.— A, External view of superior tooth of type of Triceratops serratus; B, lateral view of 

 same; C, internal view of same; D, crown view of same. Natural size. After Marsh. 



ARRANGEMENT OF THE TEETH IN THE JAW. 



In the Ceratopsia the teeth are arranged in longitudinal and vertical series after the manner 

 shown in the accompanying figures, figs. 43 and 44. The number of teeth in each vertical 

 series varies from one in the first at either end of the jaw to perhaps as many as eight near the 



