10 
Distance in a straight line from the centre (midline of skull) of the premaxillary 
border anteriorly to the posterior end of the upper premaxillary limb 380 
Distance of the posterior end of the lower premaxillary limb from the same point 
anteriorly 635 
Maximum breadth across the premaxillaries in front (estimated) 460 
Height of upper end of coronoid process above lower surface of dentary 288 
Basi-occipital (Boc.) Figures 5, 6, and 26. This is a robust,- compact 
bone as broad as long but with a comparatively sHght depth. As the 
posterior member of the basicranial axis it is in contact supero-posteriorly 
with the exoccipitals, and anteriorly with the basisphenoid. Supero- 
anteriorly its boundaries are not seen but it comes in contact here with 
the prootic and no doubt also to some extent with the opisthotic. It enters 
largely into the formation of the occipital condyle, bounds the foramen 
magnum below, and, in advance of this opening its upper surface forms a 
large portion of the floor of the brain-cavity beneath the medulla oblongata. 
It supplies aboiit five-sevenths of the articular surface of the occipital 
condyle, the exoccipitals between them providing the remaining two- 
sevenths. Its contribution to the condyle includes more than one-half 
of its lower surface. Viewing the bone from below, it is narrowest in 
advance of the condyle, broadening again to its contact with the basi- 
sphenoid where its breadth equals that of the condylar part. In front of 
the broad convexity of the condyle the lower surface of the bone is trans- 
versely concave, and on either side anteriorly a rugose but not particularly 
prominent tubercle is developed whose roughness and tumidity is con- 
tinued on to the basisphenoid. The contact of the basi-occipital and the 
basisphenoid is seen in the paratype toward either side of the lower surface, 
and can be traced upward externally for a short distance. Posteriorly 
the basi-occipital extends back for some distance beyond a point vertically 
below the roof of the foramen magnum. 
Measurements of Basi-occipital of Edmontosaurus, Specimen Cat. No. 2289. 
Mm. 
Breadth inferiorly in posterior half’ (across condyle) 98 
Breadth inferiorly at junction with basisphenoid, about 102 
Length, inferiorly, about 100 
Thickness (depth) at centre of basisphenoid contribution to occipital condyle, 
about 30 
Thickness (depth) at midline in front, about 36 
Exoccipital (Exo.) and Opisthotic (Opot.). Figures 5, d, 7, and 26. 
The exoccipitals, the lateral elements of the occipital segment of the 
cranium, form the side walls of the brain-cavity in its most posterior part, 
bound the foramen magnum on either side, and contribute largely to the 
occipital condyle. They are closely united, without trace of suture, to 
the opisthotics with which bones they evidently effected an early and 
thorough fusion. In their completion of the occipital condyle externo- 
posteriorly they project freely backward and are convex inferiorly and 
externally; they are angulated behind, and shallowly concave internally 
where they form upward and backward extensions of the floor of the 
foramen magnum. 
In the occiput the exoccipitals are thought to have been separated 
partly, if not wholly, by the supraoccipital, but in the absence of sutures 
defining the limits of the latter bone nothing definite can be said in this 
