30 - THE ROYAL SOCIETY OF CANADA 



The roof and side-walls of the main chamber are rather smooth 

 in front, more particularly near the midline of the roof, but elsewhere 

 they are roughened to some extent by short irregularly disposed 

 grooves. The shape of the chamber gives little information regarding 

 the relative size of the main divisions of the brain behind the olfactory 

 lobes, but apparently the cerebral hemispheres were well developed and 

 of fair size, leaving a rather restricted space, as regards length, for 

 the optic lobes, cerebellum, and medulla oblongata. Posteriorly 

 the roof of the main chamber drops suddenly to the foramen magnum. 

 At the beginning of this descent, in the midline, is a small excavation, 

 directed obliquely upward and backward, which may indicate the 

 position of the uppermost part of the cerebellum. Two other de- 

 pressions, or openings, one on either side of, and at a slightly lower 

 level than the one in the midline, may also represent processes of the 

 cerebellum. 



In comparison with the long, attenuated brain of Stegosaurus^ 

 that of Stegoceras is strikingly broad and short. 



Entering into the formation of the brain-case laterally, as par- 

 tially preserved in specimen 138, are a presphenoid, orbitosphenoids, 

 alisphenoids, periotic bones, not distinguishable separately, and 

 exoccepitals. 



Presphenoid. This is a fairly large bone, in its advanced position 

 as part of the basicranial axis, extending up on either side to form the 

 lateral wall of the brain-cavity external to the junction of the olfactory 

 lobes with the cerebrum. It unites above with the frontals and 

 posteriorly with the small orbitosphenoids. Interno-superiorly, a 

 little behind its midlength, its lateral upward extensions approach 

 each other closely above and are separated only by a narrow frontal 

 surface in the roof of the brain-cavity. Its external surfaces are 

 longitudinally concave. Antero-inferiorly in the right half of the 

 presphenoid of the specimen there is a suturai surface on which is 

 preserved a remnant of bone which may have belonged to the para- 

 sphenoid. Inferiorly behind this suturai surface the bone is broken 

 away so that no information is available as to its shape below. 



In the specimen all of the left side of the presphenoid is missing 

 except a small posterior marginal piece adherent in place to the frontal 

 near the midline above. In front of this a rugose surface is preserved 

 on the under side of the frontal representing the greater part of its 

 suturai union with the presphenoid. 



1 Smithsonian Institution, U.S. National Museum, Bull. 89, 1914, Osteology 

 of the Armoured Dinosauria in the United States National Museum, with special 

 reference to Stegosaurus. 



