216 PROCEEDiNGS OF THE NATIONAL MUSEUM. vol. xxxvl. 



externally by the inner surface of the supraorbital. It is not cer- 

 tainly known Avhether the external border was entirely closed by the 

 development of a j^ost-supraorbital as found in Iguanodon hernissar- 

 tensis. So far as I am aware, Camptosaurus is the only American 

 dinosaur having such a fossa. When skulls of Laosaurus and 

 Dryosaiirus are known, similar openings wall probably be found. 



I nf ratemporal fossa. — The infratemporal fossa is bounded above 

 by the posterior branch of the postfrontal; behind by the descending 

 process of the squamosal and ascending process of the jugal ; below 

 by the jugal, and in front by the ascending and descending process 

 of the jugal and postfrontal, respectively. Proportionately this fossa 

 has no such development as is found in Iguanodon., where it is greatly 

 elongated dorso-ventrally. 



There are no jjreorbital fossae. 



Orhltal carities. — As indicated by the extent of the orbital surfaces 

 on the postfrontals of Cat. No. 5473, U.S.N.M., these cavities must 

 have been of good size. Their exact contours, however, are somewhat 

 problematical, as none of the cranii studied have the boundaries of 

 the orbits intact, but, as interpreted, the reconstructed skull (see o, 

 fig. 2), which was drawn after a careful study of all available 

 material, is believed to be a fairly accurate representation of their 

 shape and size. They are bounded above on the outer margin by the 

 supraorbitals and postfrontals; behind by the descending process of 

 the postfrontals and ascending process of the jugals; below by the 

 jugals; in front by the jugals, lachrymals, and supraorbitals. There 

 is no indication of a postorbital in Camptosaurus. 



The narial opening. — The narial opening is w^ell shown in No. 1887, 

 Yale Museum (see Plate 7), after which this region of the figured 

 skull was drawn. It is of good size, suboval in outline, with its 

 greatest diameter inclined to the longer axis of the skull, as in 

 Iguanodon hernissatiensis. Excepting the posterior border, which 

 is formed by the nasals, the remainder oi the orifice is inclosed 

 bv the premaxillaries. Anteriorly, the roughened ascending process 

 of the premaxillaries roof over somewhat the lateral openings. 



Lesser foramina. — The well preserved posterior portion of the 

 skull of Cat. No. 5473, U.S.N.M., shows with unusual clearness the 

 smaller foramina of this region (see figs. 4 and 5). By a comparison 

 with the foramina in other reptilian skulls of both fossil and recent 

 forms, and by examining the relations of the various foramina to 

 one another, it is believed they have been determined with a consider- 

 able degree of accuracy. 



Beginning with the most posterior, we find on the lower lateral 

 margin of the exoccipital a pit-like depression, from the bottom of 

 which two foramina pierce the exoccipital, entering the brain case 

 just within the external opening of the foramen magnum (see XII, 

 figs. 2 and 5). In passing through the wall they diverge somewhat, 



