75 
the nasals, that is along the whole of the latter’s length. A long, narrow 
vacuity in the crest occurs between the nasals and the lower premaxillary 
expansion. 
A broad, shallow groove runs obliquely upward and slightly backward 
across the lower portion of the premaxillary a short distance in advance 
of the anterior end of the jugal. This groove was considered to mark the 
back termination of the lower part of the premaxillary (‘dower limb of 
the premaxilla”) in the original description of the skull of Stephanosaurus. 
What was then named prefrontal is now clearly seen to be the greatly 
expanded postero-inferior part of the premaxillary as the structure of the 
bone is continuous across the groove. The prefrontal is small and assists 
in the formation of the orbital rim between the lachrymal and the post- 
frontal. The small frontal is excluded from the orbital rim by the pre- 
frontal and postfrontal, and is well under the anterior, lower surface of 
the naso-premaxillary process. 
In the light of our increased knowledge of the structure of the skull 
of Stephanosaurus the original description of the skull of Corythosaurus 
Brown can be amended in certain particulars. What was called prefrontal 
(Figure 391) is now clearly seen to be premaxillary. The bone above the 
orbit determined by Brown as frontal is prefrontal. The frontal is small, 
as in Stephanosaurus, and is hidden beneath the crest. What is named 
frontal above the postfrontal and squamosal and forming the lower, hinder 
border of the crest is nasal. The front part of the crest is not nasal but 
the prolongation upward of the premaxillary. If the suture in the anterior 
part of the crest in Brown’s figure of Corythosaurus, marking the back 
termination of the nasal (premaxillary) be correct then the whole of the 
crest above the narrow central vacuity may be nasal. If this line be not 
a suture then the premaxillaries form the upper part of the crest some- 
what as in Stephanosaurus. 
In the Stephanosaurince the premaxillaries separate the external nares 
and nasal passages, the latter being enclosed outwardly by an upward 
growth of the fioor of the passage and a downward bend of the roof of 
the same. 
In Stephanosaurus the nasal passages lead up into the front part of 
a large double chamber within the crest, the entry being made at about 
the midheight of the chamber on either side of a vertical median septum. 
This nasal chamber occupies the greater part of the crest within, is flanked 
outwardly by the premaxillary and nasal bones and is somewhat over 
150 mm. in height, and narrow from side to side, with a fore-and-aft diameter 
about three-fifths the height. An exit from the chamber is present below 
posteriorly leading downward. The position of the chamber is indicated 
externally by the greater convexity of the crest laterally in an area sur- 
rounding the narrow central vacuity. The Stephanosaurinid form of the 
external nares of Corythosaurus and Cheneosaurus point to the presence 
of a nasal chamber in these genera also. 
It may be stated here that the determination of the nasal bone in 
Stephanosaurus rests not only on the evidence of the skull shown in Figure 
39H, but also on that of fragmentary stephanosaurinid crania in W}^ich 
the sutural surfaces for the nasals are preserved in complete frontals. 
In the brain-case of the skull of Stephanosaurus (Figure 39H\ 
sutures can be readily traced between the presphenoid and orbitosph 
and between the orbitosphenoid and alisphenoid. Also the suture b ’ 
etween 
