6 
The upper part of the bone, anterior to its division into upper and lower 
limbs, is thickened and folding is less pronounced. The upper and lower 
parts do not meet, but there is a narrow slit above the narial passage back 
to the diagonal groove. In C. casuarius and C. intermedins and in Lambe- 
osomtus these edges meet in their posterior portion and in Cheneosaurus and 
Hypactosaurus they are well united from near the anterior end. The floor 
and that part of the bone that is folded to surround the narial passage are 
very thin throughout, but the narial passage is completely enclosed in 
bone except for the narrow slit above referred to. 
From where the premaxilla divides the lower limb broadens and con- 
tinues backward and upward at about the same angle as the anterior por- 
tion. Its anterior edge rises somewhat steeply, and is in contact with the 
posterior edge of the upper limb for some distance, then turns sharply 
backward and presents a free edge to its posterior extremity. Throughout 
most of its length the lower limb is in contact infero-internally with a 
forward development of the nasal. Postero-inferiorly it meets the antero- 
extemal tip of the nasal. It is in contact infero-externally with the lach- 
rymal and prefrontal and is well removed from the orbital rim. This lower 
limb, with the nasal, covers the large air reservoir within the hood, but not 
the direct narial passage. 
The upper limb turns upward with a moderate angle and continues 
backward to the superior tip of the crest. It is thin internally where it 
meets its fellow on the mid-line of the skull. The internal face is quite 
deep back to opposite the anterior point of the nasal (See Plate I, figure 1). 
From here it continues back as a thin process about 50 mm. high. This 
process is flanked by the very thin superior portion of the nasal and, with 
it, makes up the thin superior part of the hood. Antero-extemally 
the upper limb folds outward and downward to form the gently rounded 
anterior edge of the hood and cover the central portion of the narial 
passage. 
A cross-section of the left premaxilla (Figure 1) taken at the posterior 
extremity of the open slit, or just about where the bone divides into upper 
and lower limbs, shows the upper and lower air passages. On the external 
side the groove that runs diagonally upward and backward across the 
lower limb is seen. The lower air passage (L.P.) is roughly oval and 
higher than wide. Behind this point it becomes slightly enlarged, then 
turns sharply forward and enters the upper air passage (U.P.) . The section 
intersects the upper air passage near its anterior extremity and shows it 
much smaller than it is farther back. A short distance behind the break 
one could see the bone dividing the upper air passage proper and the 
entrance to it into which the lower air passage extended. If the section 
had been a little farther back the S-shaped tube would have been shown, 
but the bone is too delicate to attempt sectioning except at a break. 
The nasal is far removed from the anterior naris, but surrounds the 
narial passage posteriorly. It is situated directly above the orbit and 
rests on the frontal, which is developed forward and downward to give 
greater sutural contact. It unites with its fellow on the mid-line, but they 
are separated superiorly by the thin flanges of the upper limbs of the pre- 
