[lambe] cretaceous GENUS STEGOCERAS 29 



second and larger facet is probably for a supra-orbital somewhat as in 

 Stegosaurus stenops Marsh^ which however has two supra-orbitals. 



Prefrontal. A separate bone, Cat. No. 1914, referable to Stego- 

 ceras, and included in the collection of 1915 from the Belly River 

 formation on Red Deer river, is apparently a right prefrontal but 

 belonging to an individual larger than the one represented by the 

 specimen Cat. No. 138. It has an external free border which formed 

 part of the orbital rim supero-anteriorly. Apart from its general 

 shape and suturai facets, its connection with and position in the orbit 

 is indicated by its under surface which is concave and has the peculiar 

 foraminal markings observed in the surfaces supplied to the roof of 

 the orbital cavity by the frontal and postfrontal. As in the post- 

 frontal the foramina, opening very obliquely to the surface, point to- 

 ward the frontal. If this bone is a prefrontal the orbital rim between 

 it and the postfrontal is completed by a single supra-orbital bone of 

 comparatively small size at the middle of the upper curve of the rim 

 and separated from the frontal by the postfrontal. 



With the exception of a specimen, Cat. No. 2370, which may 

 prove to be a premaxillary, no cranial elements are known in Stegoceras 

 from in advance of the frontals and prefrontals but it is presumed that 

 the facial portion was short and obtusely pointed giving to the skull, 

 in superior aspect, a triangular outline of which the length was prob- 

 ably considerably less than twice the posterior breadth. 



The brain-cavity of Stegoceras is wide in comparison with its 

 length. Compared with the size of the skull it is small, having a total 

 length equal to only one-half of the breadth of the skull at the parieto- 

 frontal suture. As preserved in specimen Cat. No. 138 it consists of a 

 main chamber, occupying the posterior two-thirds of its length, and 

 an anterior pair of transversely placed concave surfaces which were 

 over the olfactory lobes. The main chamber, of which the roof and 

 most of the upper part of the side-walls are preserved, is 22 mm. long 

 and 20 mm. in maximum width slightly in advance of its midlength. 

 It contracts to either end, in the form superiorly of a prolate spheroid, 

 passing posteriorly down into the foramen magnum, and expanding 

 anteriorly into the broad, short olfactory surfaces which have a length 

 of 15 mm. and a combined breadth of 16 mm. The olfactory tract, 

 broadens forward, ends squarely in front, and has a median ridge, 

 most developed anteriorly, separating the two surfaces which in their 

 areal extent lie in advance of a line passing through the centre of the 

 roof of the orbit on either side. 



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

 armoured dinosauria in the United States National Museum, with special reference 

 to the genus Stegosaurus, by Charles W. Gilmore, 1914. 



