32 BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



the antero-external part of the dorsal surface is concavely beveled for the anticu- 

 ation with the overlying portion of the prefrontals (ps., fig. 1, pi. 10). The postero- 

 external angles of the frontals are much thickened and are roughly beveled for sutural 

 union with the anterior internal process of the postorbital. As mentioned previ- 

 ously, in S. stenops, the frontal is excluded from the supratemporal fossa, but reaches 

 it in a skull of S. armatus, No. 4936. (See fig. 1, pi. 10.) The posterior ventral 

 surface is concave transversely and forms the roof covering the olfactory lobes 

 of the brain. The orbital margin is horizontally concave. 



Prefrontal (pf.). — The prefrontal, as shown in specimen No. 4934, is an 

 elongated subelliptical bone which overlies'the superior surface of the frontal and 

 nasal bones at their junction (pf., pi. 6). The outer border articulates with the 

 postfrontal and presupraorbital bones and more anteriorly it meets the lachrymal. 

 The posterior end is separated from the anterior process of the postorbital by an 

 intervening part of the frontal measuring 15 mm. on the superior surface. 



Postfrontal (po.f). — The postfrontal, viewed from above, is an irregularly 

 shaped element which completely fills the space between the external supraorbital 

 bones and the postorbital and prefrontal, and frontal (see po.f, pi. 6 ). The dorsal 

 surface of this element is rugosely roughened, and it unites with the frontal by a 

 comparatively short sutural edge. I was at first inclined to regard this element 

 of the skull as a supraorbital bone, but I am indebted to Dr. R. Broom for first 

 pointing out to me that it should be identified as the postfrontal, and that the 

 so-called postfrontal, postorbital complex represents only the postorbital. That 

 this is the proper relation of these elements appears to be indicated by a very 

 similar arrangement found in the disarticulated skull of a small Ceratopsian, 

 Brachyceratops montanensis 1 Gitmore, recently acquired by the National Museum. 



Postorbital (po.). — The postorbital is a triradiate bone with one short, stout 

 process, extending inward and uniting with the frontal and parietal; a second 

 posteriorly directed branch unites by squamous suture with the outer surface of 

 the squamosal and forms the upper temporal bar; the third, the longer one of the 

 three, is a descending process which overlaps externally an upgrowth from the 

 jugal and thus forms a postorbital bar separating the orbit from the infratemporal 

 fossa. The latter process, as in Camptosaurus, is trihedrial in cross-section. It 

 descends to the lower border of the orbit (see po, pi. 5), not part way as in Campto- 

 saurus, Tracliodon, and Iguanodon. On the external surface at the junction of the 

 three branches is a roughened protuberant area over which the hollowed out 

 opposing surface of the postsupraorbital is attached. The postorbital as here 

 identified, is in part the postfrontal of Marsh. 



Prootic (pro.). — The prootic is not clearly differentiated in any of the skulls 

 before me, although its approximate position and relationships can be determined. 

 As in Crocodylus and- Camptosaurus, this bone fills the area between the basisphenoid 

 below, the alisphenoid in front, and the parietal and supraoccipita^l above, and in 

 all probability the opisthotic behind, although the prootic and opisthotic are so 

 fused as to be unrecognizable as separate elements. The foramen ovale in the 

 alligator and in Camptosaurus is between the prootic and alisphenoid, the larger 

 part of it being in the prootic. This foramen occupies a similar position in Ste- 



' Smiths. Misc. Coll., vol. 63, 1914, p. 5, pi. 2. 



