30 BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



finished surface, wliicli was probably covered by cartilage. In very old individ- 

 uals these bones sometimes coalesce, as is shown by specimen No. 6645 (fig. 5); 

 On the posterior surface is a blunt vertical median ridge extending from the upper 

 border of the foramen magnum to the top of the bone. On the latero-ventral 

 angles thin processes are developed, which extend outward and backward and lap 

 along the exoccipitals. The posterior surface is inclined forward from the ver- 

 tical at an angle of 35°. Ventrally it articulates by wide, oblique, heavy sutural 

 surfaces with the exoccipitals (fig. 4). The anterior surface is deeply excavated and 

 forms a considerable part of the posterior boundary of the brain case. On the 

 anterior lateral borders, well shown in specimen No. 6645, are notches which reach 

 the exterior by an opening between the supraoccipital and parietal. These would 

 appear to correspond with those noted by Hay ^ in the brain case of Triceratops. 

 They probably transmitted blood vessels, for no nerves are known to leave this 

 part of the brain. Similarly placed foramina are present in the supraoccipital (Cat. 

 No. 5692, U.S.N.M.), described and figured by Lull ^ as belonging to Pleurocoelus 

 nanus Marsh. In this connection it might be well to call attention to the close 

 resemblance of this bone to the supraoccipitaLs of Stegosaurus and Camptosaurus, 

 more particularly to the latter. That it pertains to a member of the Ornithopoda 

 I am fully convinced and, in all probability, should be referred to the genus Dryo- 

 scuurus, as being the only representative of the Camptosauridae known to occur in 

 the Arundel formation. 



Basisphenoid (bs.) and Presphenoid {p. sp.). — The basisphenoid is a short bone 

 which unites posteriorly with the basioccipital and dorso-laterally with the prootics, 

 alisphenoids, and slightly, if at all, with the exoccipitals. The median ventral sur- 

 face is deeply concave transversely, and on the latero-anterior end a pair of diverg- 

 ing processes are given off, which extend backward and downward, with surfaces in 

 front near their extremities for union with the pterygoids (pi. 7). The dorsal surface 

 is continuous with the basioccipital and completes the floor of the brain case. The 

 pituitary fossa is deep and extends backward somewhat beneath the floor of the 

 median vesicle (pi. fig. 3, pi. 10). At its bottom and on either side are short fora- 

 mina, which lead outward and backward, opening externally into vertically elon- 

 gated slits on the sides of the basisphenoid (c. fig. 10). These serve to transmit 

 the carotid arteries. 



Anterior to tnis fossa the bone contracts rapidly to a narrow-pointed process, 

 which probably represents the persphenoid, although its complete anterior extent 

 can not be determined from available material. (See p.sp. fig. 10.) 



Alisphenoid^ {al. sp.). — The alisphenoid is a small subtriangular bone (fig. 10) 

 resembling very closely the corresponding element in Camptosaurus.* Specimen 

 No. 4935, upon which the description to follow is based, shows the alisphenoids 

 detached, but in aU other skuUs they are so closely coossified as to be almost indis- 

 tinguishable. DorsaUy it unites with the parietal, frontal, and to a slight degree 

 with the postorbital. The outwardly turned anterior end is received in a trans- 

 verse notch on the lower and outer side of the frontal. The lower posterior border 



1 Pro=. U. S. Nat. Mus., vol. 3^909, pp. 103. 



= Maryland Gejl. Survey, Lo5^FCreta:eous, 1911, p. 192, pi. 15, flgs. 3a-c. 



* In part the orbitosptienoid of H. F.Osborn, Mem. Amer. Mus. Nat. Hist., new ser., vol. 1, pt. 1, p. 5. 



I C. W. Gilmore, Proc. U. S. Nat. Mus., vol. 36, 1909, pp. 208-209, fig. 5. 



