18 THE MORPHOLOGY OF THE SKULL 



rugosities. The suture between the preinaxillary and maxillary terminates below in 

 the middle of the diastemal notch. Superiorly the premaxillaries send upward and 

 backward long processes, which pass between the nasals and form the upper portion 

 of the nares. The premaxillaries always carry large tusks and smaller teeth ; the 

 tusks lie near the median line in the fore part of the bone, but their number seems to 

 be variable in the different species. 



The maxillaries : The maxillaries are peculiar in their great vertical extent 

 forming the greater portion of the elevated facial region. The upper portion is 

 remarkably thin, never exceeding 2 mm., even in tlie largest specimen, while the edge 

 of the bone carrying the teeth may reach a thickness of two and three centimeters. 

 The thinness of the upper portion of the maxillary is shared by the adjacent bones, 

 the nasals, prefrontal, jugal and lachrymal ; so that this part of the skull is almost 

 always shattered in the j^rocesses of fossilization and lost. Specimen 1002 is the only 

 one I know in which the facial region is perfect. The lower edge of the bone is very 

 abruptly widened into a thick dentigerous border, Figs. 4 and 5, which is in strong 

 contrast to the weak upper portion of the facial region. The width of this border is 

 greatest opposite the enlarged canine near the anterior end of the maxillary and 

 decreases in width toward the posterior end of the bone as the teeth become smaller. 

 In the diastemal notch there seems to be no great widening of the edge, even in the 

 forms where teeth are present in the notch. The posterior end of the bone articulates 

 with the jugal, as described above. The outer surface of the bone on the lower edge 

 is marked with pits and rugosities. 



The teeth are lenticular in form with distinct fore and aft cutting edges which 

 are strongly serrate. The roots of the teeth are implanted in distinct sockets which 

 may reach a depth as great as the length of the tooth beyond the outer edge of the 

 bone ; the outer edge of the bone extends much farther down than the inner so that 

 a good bit of the length of the tooth after it leaves the socket rests against this edge. 

 The root of the tooth is hollow and its inner end is open so that it is evident that the 

 teeth were replaced by alhsorption of the root and continued growth of new teeth ; 

 this process is seen in actual progress in some places. In specimen 114 there are two 

 large canines in the maxillary and in the others but one, this is possibly a case of 

 where one canine has failed to fall out as the other develops. The number of maxil- 

 lary teeth is variable but does not exceed twenty in any of the specimens. Teeth 

 develop in the diastemal arch in some forms of the Pelycosauria and not in others, 

 but this seems to be a developmental feature, as teeth occur in the more primitive 

 Diopcm, in the notch but are absent in Dimetrodon and Naosaurus, the most 

 specialised. 



