51 
strated, 1 is composed entirely of the backward prolongation of the com- 
bined premaxillary and nasal bones. The facial slope of the crest in this 
specimen is steep, but more receding than in either Corythosaurm casuarius 
Brown or Lambeosaurm lambei Parks, and resembles more nearly in this 
respect the skull of Corythosaurm excavatm Gilmore in the University of 
Alberta collections. With the tooth rows in an horizontal position, the 
crest reaches its highest elevation at a point slightly posterior to the mid- 
line of the orbit. From the apex posteriorly it presents a convex outline 
that curves rapidly downward. Unlike Corythosaurm the posterior 
termination does not overhang the occiput of the skull. Throughout its 
extent the lateral halves of the crest are distinctly separated by a median 
suture, and this suture is continued forward to the tip of the beak. 
Figure 13. Skull of Hypacrosaurus oltispinus Brown. No. 8501, Geol. Surv., Can. Viewed 
from the right side. Slightly restored. Dn, dentary; Fr, frontal; Ju, jugal; la, lachrymal; mx, 
maxillary; na, nasal; o, orbit; par, prearticular; prf, prefrontal; pmx, premaxillary; poc, paraocci- 
pital; pa, postorbital; PrDn, predentary; qu, quadrate; quj, quadratojugal; sq, squamosal; stir, 
surangular. About eleven-sixteenths natural size. 
The greatly expanded posterior end of the inferior branch of the 
premaxillary is a conspicuous feature of this genus, and it forms the greater 
part of the central area of the crest. The narrower superior prolongation 
of the premaxillary forms the whole of the upper anterior portion, extending 
‘Geol. Surv., Can., Mem. 120, 1920, pp. 74-76. 
