490 PROCEEDINGS OF THE NATIONAL MUSEUM vol.84 



show the foramen as entirely enclosed within the orbitosphenoid 

 bone. 



Posterior to the optic foramen and separated by a wall of bone 

 8 mm in width is the foramen for the third nerve (see fig. 34). Imme- 

 diately above the foramen for the third nerve are two small foramina 

 one above the other. On the external surface a short shallow groove 

 runs forward from each of these openings. It seems quite probable 

 that the most ventral gave exit to the fourth or trochlear nerve. 

 The superior one may have transmitted a blood vessel. On the 

 ventral posterior border of the orbitosphenoid there is a shallow 

 groove leading down to the sutural border which in the articulated 

 skull may have led to the foramen for the abducent or sixth cervical 

 nerve. 



MEASUREMENTS OF ORBITOSPHENOID 



Greatest length, anteroposteriorly 34 mm 



Greatest height, dorsoventrally 48 mm 



Ethmoid. — A pair of small subrectangular elements (see fig. 34) are 

 identified as the ethmoids. In adult skulls the suture between the 

 ethmoid and orbitosphenoid becomes so fully coalesced as to leave no 

 trace of their union. In Tyrannosaurus Osborn ^ indicated the ques- 

 tionable presence of an ethmoid, and in describing the skull of Edmonto- 

 saurus Lambe ^ designated the lateral area immediately posterior to 

 the exit of the olfactory nerves as being the presphenoid. In a brain 

 case of Saurolophus osborni illustrated by Brown,^ it now becomes 

 evident, in the light of the present specimen, that the anterior portion 

 of the element designated alisphenoid is the coalesced alisphenoid and 

 orbitosphenoid and that the bone called presphenoid is the ethmoid. 

 Other authors have considered all the brain case between the ali- 

 sphenoid and exit for the olfactory nerves as being the orbitosphenoid 

 bone. After comparison of the present specimen with the brain case 

 of Antrodemus, Camarasaurus, Triceratops, Stegosaurus, Thespesius, 

 and Kritosaurus, 1 am of the opinion that the ethmoid, although fused, 

 is present in all these genera and probably in all Dinosauria. 



Posteriorly the ethmoid unites with the orbitosphenoid, being 

 received in a groove along the edge of the latter bone, which sends a 

 wide thin process forward for a squamous overlap for one-half the 

 width of the ethmoid, as shown in figure 34. In the left element this 

 external sutural surface covers more than half the width of the bone. 

 The strongly ridged and radiating nature of the suture renders the 

 union of these two elements distinctive and thus contributes to the 

 positiveness of their proper identification. The upper sutural end 

 is widened transversely but constricted anteroposteriorly and thus 



•Osborn, H. F., Mem. Amer. Mus. Nat. Hist., vol. 1, no. 1, figs. 8, 12, 1912. 



' Lambe, L. M., Qeol. Surv. Canada Mem. 120, p. 47, fig. 26, 1920. 



• Brown. Barnum, Bull. Amer. Mus. Nat. Hist., vol. 31, p. 134, fig. 3, 1912. 



