OSTEOLOGY OF THE AEMOEED DINOSAUEIA. 41 



postorbital bar formed by the united processes of the postorbital and jugal; ven- 

 trally by the quadratojugal and quadrate; and posteriorly by the quadrate. 



Orbital cavities (h.) . — The orbits are of large size, suboval in outline, the greatest 

 diameter being antero-posteriorly (b., pi. 5). The length of the orbit is one-fifth of 

 the total length of the skull. Viewed from the side it is bounded above by the 

 supraorbital bones; below by the jugal; anteriorly by the lachrymal and presupra- 

 orbital; and posteriorly by the postorbital bar as explained above. Appaiently 

 there were no sclerotic plates developed for the protectio"n of the eye. There are no 

 pre- or supraorbital fossae in Stegosaurus, as found in Camptosaurus and Igvxinodon. 



Anteriornares (a.). — The external narial opening is of good size, subelliptical in 

 outline, with its greatest diameter parallel to the longer axis of the skull. Except- 

 ing the posterior, and part of the upper boundary formed by the nasals the rest of 

 the orifice is enclosed by the premaxillae. 



Posterior nares. — On the median posterior junction of the pterygoids in speci- 

 men No. 4934 is a single large foramen which apparently represents the posterior 

 naiial opening. The crushing together of the bones anterior to the foramen ren- 

 ders it impossible to trace its course and thus determine its function beyond a 

 question. While it is well within the pterygoids, the basisphenoid forms the upper 

 boundary (pi. 7). It would appear from this specimen that the posterior nares 

 in Stegosaurus occupies even a more posterior position than in the recent Crocodiha 

 where the posterior nares are wholly inclosed by the pterygoid bones. From 

 Hatcher's description ' it would appear that the posterior nares in the Ceratopsia 

 occupy a somewhat similar position in relation to the bones of the palate. 



Palatine vacuities. — The crushed and damaged condition of this region of tlie 

 skull precludes the posssibihty of bounding these openings. 



Foramen magnum (fm.) . — Taking into coiisideration the very small size of the 

 brain in Stegosaurus, the foramen magnum is comparatively large. The size of 

 this opening, however, varies considerably in different individuals as may be 

 readily seen by comparing figures 4 and 5. In most skulls it is ovate in outline 

 with the greatest diameter transverse (pi. 9, fig. 2,fm.). In specimen No. 6645 the 

 greatest diameter is vertical. As in Camptosaurus it is bounded below by the 

 basioccipital, laterally by the exoccipitals, and above by the supraoccipital (fig. 4). 



Lesser foramina. — None of the skuUs in the National Museum collections are 

 sufficiently well preserved to show clearly the relationships of all of the smaller 

 foramina. However, the five cranii at hand supplement one another to such an 

 extent, that the more important openings have been recognized, and by compar- 

 ing them with other reptihan skulls of both recent and fossil forms, it is beheved 

 they have been identified with a coiisiderable degree of accuracy. 



Begummg with the most posterior, it is found that the exoccipital is pierced 

 on its lower external side by two small fcframina, one in front of the other, the 

 posterior one being located somewhat more dorsaUy. The latter represents the 

 exit of the twelfth or hypoglossal nerve (XII, fig. 10). It enters the brain case 

 just withm the external openmg of the foramen magnum. The next anterior 

 foramen occupies a position essentially that of the anterior condyloid foramen in 



1 Mon. 49, XT. S. Gcol. Surv., 1907, p. 36. 



