OF THE PELYCOSAURIAN GEXrS DBrETRODON 



17 



is bent at right angles, so that the lower half forms the floor of the posterior half of 

 the nares and the upper half its posterior edge. The two bones of the opposite side 

 meet in the median line. Of the vertical portion, the inner part is only one-half so 

 high as the outer, so that while the outer part extends to the top of the nares, the 

 inner part reaches up only one-half the height. This forms a dam across the posterior 

 part of the nares, so that the air in entering must first pass upward and over the dam 

 and then downward into the mouth. On the outer side of the septo-maxillary a short 



Fig. 4. Cross section through the facial region of Fig. 5. Section of same opposite the middle of the 



the skull of D. gigas, No. 1002, opposite the middle of the diastemal notch. Letterings as in Fig. 4. 



palate. Showing the thinness of the facial bones and the 

 alveolar edge. »., nas;il ; mx., maxillary ; pt., vertical 

 plates of pterygoids ; pi., palatines ; pi\, prevomer. 



process at the posterior inferior angle of the nares divides two foramina which pass 

 between the septo-ma.\il]ary and the maxillary to the interior of the skull. Their 

 function is entirely problematical. 



The lyrcmaxillarics : The premaxillaries are heavy rounded bones uniting in the 

 median line by a wide sutural area. The lower edge is thickened for the reception of 

 the tooth sockets, and the outer surface of the edge is marked by deep pits and 



