[lambe] cretaceous GENUS STEGOCERAS 31 



Orhito sphenoid. Information regarding this element is derived 

 in specimen 138 from the right side principally and that only from 

 the upper part of the bone as it is imperfect below. The left orbito- 

 sphenoid is represented only by a remnant behind what remains of 

 the left presphenoid. This element is narrow, and small in com- 

 parison with the presphenoid. It is wedged in between the latter 

 bone in front and the alisphenoid behind, and meets the frontal 

 narrowly above. Its sutures are very distinctly preserved in the 

 specimen as shown in plate I. 



Alisphenoid and exoccipital. Behind the orbitosphenoids are the 

 alisphenoids which unite with the parietals and frontals above. 

 Supero-laterally they extend outward in the direction of the post- 

 orbital bar as far as the parieto-postorbital suture. Posteriorly their 

 boundary is not seen there being apparently a coalescence of the 

 bones enclosing the main chamber of the brain-cavity laterally as far 

 back as the foramen magnum. The part on either side of and above 

 the foramen magnum is regarded as belonging to the exoccipital, the 

 intermediate portion being composed of the periotic bones. The 

 suturai surface separating this alisphenoid — otic — exoccipital com- 

 plex from the parietal above appears distinctly both within the brain- 

 cavity and externally. Within the brain-cavity the suturai lines 

 meet posteriorly in the median invagination in the roof which has been 

 already referred to as possibly indicating the position of the cere- 

 bellum. Externally the suturai lines continue inward across the 

 occiput, meeting some distance above the foramen magnum, and 

 marking the union of the exoccipitals with the lower end of the supra- 

 occipital. Neither within the brain-cavity nor in the occiput is the 

 junction of the exoccipitals, above the foramen magnum, detected. 

 These elements, enclosing the brain-cavity laterally behind the 

 orbitoshpenoids, are imperfect below, being broken off above the 

 cranial foramina. None of the nerve exits is seen, in this specimen, 

 as the brain-case is not preserved sufficiently far down to include them. 



Semicircular canals. The otic mass or complex forms a convex 

 surface internally which, protruding into the brain-cavity, reduces 

 the width of the cavity below the level of the cerebellum. In the 

 broken surface of the complex is indicated on each side the course 

 of the anterior and posterior semicircular canals. The horizon semi- 

 circular canal curves outwardly within the bone and cannot be seen. 

 In the figure, plate I, the position of the canals is shewn by the 

 two small round openings leading up from the broken surface of the 

 otic mass near the letters Ot. On the other side the surface of the 

 bone within the brain-cavity has broken away and the two canals are 

 seen meeting above in a curve which is foreshortened in the figure. 



