SKULL OF A HADROSAURIAN DINOSAUR — GILMORE 487 



Postorbital. — The element here designated postorbital is usually 

 called postfrontal by authorities. It may represent a complex of 

 these two elements, as in the theropodous dinosaurs, but in the present 

 instance the alisphenoid articulates with it on the internal side, being 

 received in a depression or pit. In those dinosaurian skulls in which 

 prefrontal, postfrontal, and postorbital bones can be distinctly recog- 

 nized, this cupped depression for the alisphenoid is always on the 

 inner side of the postorbital bone, and it is largely for that reason 

 that it is so designated here. 



The postorbital has the usual triradiate form. Its posterior exten- 

 sion overlaps by squamous union the forward extension of the squa- 



Figure 33.— Articulated postorbital and prefrontal bones (U.S.N.M. 

 no. 11S93), lateral view. Ju, Process that unites with jugal; A'a, inner 

 side in contact with the nasal; Po, postorbital; Prf, prefrontal; Sg, 

 process that unites with the squamosal. One-half natural size. 



mosal and forms the supratemporal arcade separating the supra- 

 temporal from the infratemporal fossa. The posterior end is expanded 

 dorsoventrally and on the inner side is deeply excavated for the 

 squamosal process, which extends forward the full length of this bar. 

 The anteriorly directed process is in contact internally by a zigzagged 

 suture with the frontal for half its length. The heavy anterior end 

 unites by suture with, the prefrontal above the center of the orbit, 

 as shown in figure 33. The descending bar, which is trihedral in 

 cross section, united with an ascending process of the jugal by squa- 

 mous union to form the postorbital bar. On the inner side at the 

 junction of the three rays is a shallow rounded depression for the 

 articulation of the outer end of the alisphenoid. 



Prefrontal. — The prefrontal completes the upper border of the orbit 

 articulating behind with the postorbital and in front with the lachry- 

 mal. Its upward extension is very thin and lapped the base of the 

 elevated crest formed by the nasals as in Cheneosaurus. With the 

 postorbital it excludes the frontal participation in the orbital rim, a 

 peculiarity of the crested hadrosaurs, whereas in most of the crestless 



