22 
MUSEUM BULLETIN NO, 12. 
on the top of the nasal horn-core, and in the skull of Centrosaurus 
figured in Plate XI (Geological Survey collection of 1914) a 
separate ossification occurs at the upper termination of the rostral 
bone. Further, at the greatest elevation of the postfrontal 
above each orbit in the type specimen of Styracosaurus and also 
in that of Bmchyceratops dawsoni there is a shallow concavity 
of somewhat irregular oval outline which has the appearance 
of being a sutural surface for the attachment of a separate bony 
growth which became detached before or during fossilization, 
as was often the case with the epoccipitals and especially with 
the epijugal. This separate ossification above the postfrontal 
was evidently an incipient supraorbital horn-core and furnishes 
the clue to the origin of this horn-core in the homed dinosaurs 
generally from a centre of ossification distinct from the post- 
frontal. The supraorbital hom-core is to be regarded, therefore, 
not as a simple outgrowth from the postfrontal but as a separate 
element, in the same category with the epijugal and the 
epoccipitals and like them to become firmly attached to the 
underlying element with generally a more or less perfect 
obliteration of the sutural contact. The large posterior pro- 
jections of the frill in Styracosaurus (Plate III, figure 2, and Plate 
VI, figure 2) may be regarded as a striking example of extreme 
enlargement in separate bony growths with loss of any trace of 
a basal suture. In the supraorbital hom-core a more or less 
distinct basal engirdling groove or constriction is sometimes 
present as an indication of where coossification has taken place. 
Also foramina or deep pits not infrequently occur at the horn 
base and may be regarded as marking the position of a closed 
suture. 
The presence of epoccipitals on the margin of the neck-frill 
is distinctive of certain genera. They were developed in all 
three groups of homed dinosaurs apparently as accessories 
during specialization, to be abandoned when found useless as 
appendages to the armature. The primitive Eoceratops was 
without them, later they appear in Triceratops. In the wholly 
Belly River Centrosaurus group the specialized Centrosaurus 
has these separate ossifications in a well-developed state, Styra- 
cosaurus has a profusion of them of unusual length, while in 
