I02 



REVISION OF THE PELYCOSAURIA. 



The prefrontal forms the superior anterior angle of the orbit and extends forward 

 betweeen the nasal and frontal above and the niaxillar}- and lachrymal (?) below. 

 The posterior portion of the bone is bent at right angles on the antero-posterior axis, 

 so that the upper portion of the bones is horizontal and the lower vertical. The 

 horizontal portion forms a part of the roof of the skull and the anterior part of the 

 superorbital ridge. On the vertical portion a strong ridge carries the superorbital ridge 

 forward on to the facial region. 



Beneath the posterior end of this 



ridge and just anterior to the orbit 



is a deep pit. The presence of this 



ridge and pit is one of the charac- 

 teristic features of the Pelycosaurian 



skull. 



The nasals are elongate bones 



occupying the median line of the 



skull and extending from a point 



just anterior to the orbits to the 



anterior nares in front. 



The septo-niaxillary : Ante- 

 rior to the nasal and forming the 



posterior edge of the narial open- 

 ing is a singular bone, the septo- 



maxillary, difficvilt of description, 



but indicated in plates 17 and 18, 



figs. I. Each bone 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 pro- 

 cess at the posterior inferior angle of the nares 

 divides two foramina which pass between the septo- 

 maxillary and the maxillary to the interior of the skull, 

 problematical. 



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 rugosities. The suture 

 between the premaxillary and maxillary terminates below in the middle of the dias- 

 tema! notch. Superiorl}- the premaxillaries send upward aud backward long processes, 

 which pass between the nasals and form the upper portion of the nares. The premax- 

 illaries 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 are peculiar in their great vertical extent, forming the greater 

 portion of the elevated facial region. The upper portion is remarkably thin, never 



Fig. 30. — Section o( skull ot D. gigas. 

 No. 1002 University of Chicago, 

 opposite posterior end of palate. J*/, 

 palatine, //, slender vertical plates ot 

 pterygoid, m\, maxillary. 



Fig. 3 1 . — Cross-section of same skull through 

 diastemal notch, pmx, premaxil- 

 lary ; pi', prevomet ; ft, pterygoid ; 

 n, nasal. 



Their function is entirely 



