OSTEOLOGY OF THE AKMOEED DINOSAUEIA. 



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magnum, and, as mentioned above, contribute to tbe formation of tbe occipital 

 condyle. Viewed posteriorly they rise from heavy expanded pedestals that rest 

 laterally on the oblique lateral surfaces of the basioccipital, as shown in figure 4. 



The. upper portion develops a vertically expanded wing-like paraoccipital 

 process, which extends outward and backward at an angle of 45° to the main axis 

 of the skull, ending at a point posterior to the occipital condyle (pi. 6). Dorsally 

 the exoccipitals are united by the widely expanded supraoccipital, and the union 

 of these bones is greatly strengthened by the processes developed on the postero- 

 external angles of the supraoccipital, which lap along the upper posterior surfaces 

 of the exoccipitals, forming a longitudinal swelling of the bones at this point (pi. 9, 

 fig. 2, also text figs. 4 and 5) . The dorsal border of the paraoccipital process supports 

 the parietosquamosal processes. In specimen No. 2274 the squamosal passes from 

 the dorsal to the anterior surface near the outer end, as shown in figure 4, but in No. 

 4934 the squamosal appears to cover the entire dorsal border (see fig. 2, pi. 9). 

 The other sutures of the occipital region in this specimen are coalesced and the 

 union of the elements can not be 

 clearly determined. In specimen 

 No. 4935 the right exoccipital is 

 wholly detached and shows beauti- 

 fully the chief characteristics of this 

 bone. 



The opisthotic and exoccipital 

 in Stegosaurus evidently unite early 

 in life, for in none of the skulls is 

 there any indication whatsoever of 

 their sutural union. 



These coalesced bones unite 

 with the prootics by a long lap- 

 ping suture on the anterior surface. 

 The end of the paraoccipital process 

 is broadly rounded from above 

 downward, and in the articulated skull the lower half projects free, and hides the 

 upper end of the quadrate from a posterior view. 



The exoccipital is pierced by two small foramina. The more posterior one 

 enters the brain case just within the entrance of the foramen magnum and serves 

 as exit for the twelfth or hypoglossal nerve (XII, fig. 10). Anterior to and sepa- 

 rated by a thin wall of bone from the exit for the twelfth nerve is the anterior 

 condyloid foramen. 



Supraoccipital (so.). — The supraoccipital in Stegosaurus, as in all other pre- 

 dentate dinosaurs, 1 where the elements of the skull are known, forms the upper 

 boundary of the foramen magnum. 



Viewed from behind the supraoccipital is a comparatively wide bone, wedged 

 in between the exoccipitals and parietals (fig. 4). The median dorsal border does 

 not unite by suture with the overlying parietals, but, as in Carnptosaurus has a 



Fig. 5. — Posterior view of the skull of Stegosaurus stenofs? 

 Marsh. Cat. so. 6645, U.S.N.M. i Nat. size, to, Basioccipital; 

 Ei.o, exoccipital; fm, foramen magnum; p, parietal; p.oc, para- 

 occipital; so, supraoccipital; Sq, squamosal. 



i F. von Huene has recently shown that the supraoccipital in Trkcralirps extends to the foramen magnum. See Neues Jahr- 

 bnch, vol. 2. 1912, pp. 147, 148, flgs. 1 and 2. 



