37 
Measurements of Left Ectopterygoid of Specimen, Cat. ]Vo. 2289. 
Min. 
Estimated length 192 
Estimated maximum breadth (height) posteriorly 100 
Thickness of bone where it fills the postero-maxillary notch 23 
Average thickness of bone elsewhere 6 
Vomer. (F.). Figure 3. This element is known only from a small 
portion disclosed toward its anterior end in the specimen collected in 
1912 (Cat. No. 2288). The vomer has not been found with the disar- 
ticulated skull, belonging to the skeleton collected in 1916. It apparently, 
however, connected in front with the maxillaries on the inner side of the 
anterior maxillary processes, and behind with the pterygoids on the interno- 
superior surface on the height of their anterior wings, and had an estimated 
length of 385 mm. 
The portion of the vomer seen in the 1912 specimen is 125 mm. long, 
attains a maximum breadth of 20 mm., and lies in the midline of the skull 
in the posterior half-length of the narial opening on a level with and midway 
between the upper border of the lower premaxillary limbs. It is broadest 
at midlength, narrows slightly forward and much more so backward, 
so as to have an anterior breadth of 16 mm. and a posterior one of 6 mm. 
In front it ends abruptly in a transverse break so that the shape of the bone 
at the maxillary connexion is not revealed. Behind, it continues into the 
matrix of which as much has been removed as is at present possible. To- 
ward the front what appears to be a median line of division, traceable 
for a short distance back, suggests a coalescence of an elemental pair. 
The vertical inner surface of the laterally compressed anterior maxillary 
process in Edmontosaurus is rugose along the whole of its length in advance 
of a narrow, horizontal, shelf-like protrusion, projecting inward from the 
hinder part of the process, for the support of the vomer from beneath. 
That the vomer passed forward beyond the anterior maxillary processes 
is probable judging from the appearance of the inner surface of these 
processes which are rugosely striated to the tip. In Prosaurolophus a 
shallow groove is present on the inner side of the superior border of the 
lower premaxillary limb beneath the anterior end of the narial opening 
and just behind where the lower limbs separate for their backward ascent. 
This groove in Prosaurolophus marks the position of the attenuated anterior 
end of the vomer, and it is probable that the vomer of Edmontosaurus 
had a like slender termination in advance of the maxillae. To all appear- 
ances, therefore, the vomer remained narrow between the anterior processes 
of the maxillaries, separating them from each other by only a short trans- 
verse distance. 
The shape of the posterior termination of the vomer is unknown. 
On the inner side of the narrowly compressed ridge forming the most 
elevated part of the pterygoids forward is a roughened, transversely 
concave surface on which the posterior end of the vomer appar- 
ently lay. Whether the vomer bifurcated behind and reached the ptery- 
goids on either side of the median line in this manner, or united with the 
pair by a horizontal expansion has not been ascertained as yet. Judging, 
however, from the distance apart of the pterygoids anteriorly the vomer 
had a posterior breadth of about 50 mm. Its estimated breadth between 
the forward end of the anterior maxillary processes is about 10 mm. In 
this genus the vomer appears, therefore, to have been slender throughout 
and devoid of any considerable expansion. 
