46 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 62, 
described long-beaked squalodonts. It comprises more than two- 
thirds of the total length of the skull and was originally still longer. 
The narrowest portion is slightly in advance of the middle. The 
mesorostral gutter was open the full length of the rostrum, though 
the premaxillae approximate each other so closely at a point anterior 
to the middle that it may have been nearly closed at this point. 
Distally, this gutter is formed entirely by the premaxillae, which 
meet mesally and ventrally in a linear suture in front of the alveolae 
or the first molars; proximally, the vomer and the premaxillae con- 
tribute to its formation. The dorsal margins of the mesorostral 
gutter are formed by the overhanging edges of the premaxillae. 
In the squalodonts, in consequence of the prolongation of the snout 
into a beak, the vomer acquires a considerable length and is so placed 
as to afford support for the premaxillae as well as the maxillae, and 
on the base of the skull extends backward as far as the basisphenoid. 
The vomer disappears in the floor of the mesorostral gutter slightly 
in advance of the narrowest portion of the rostrum or above the 
alveolae for the second molars. It increases in width posteriorly, and 
in the region just anterior to the alveolae for the fifth molars takes 
part in the formation of the lateral walls. On either side it is in con- 
tact with the adjoining premaxilla, the surfaces being smoothly mor- 
tised into one another. This contact between the vomer and the 
premaxilla has its posterior limit at the anterior margin of the pre- 
sphenoid. 
The premaxillae are bowed, and approximate each other closest, 
as remarked above, at a point anterior to the middle of the rostrum. 
Just anterior to the premaxillary foramina the premaxillae commence 
to expand horizontally, and rapidly increase in width to a point 
20 mm. in advance of the mesethmoid. From this point posteriorly, 
they diminish in width, the internal margins being deflected ven- 
trally as they approach the external nasal passages. Posterior to 
the anterior margins of the nasals they send back splintlike ascending 
processes, which are lodged in deep paired grooves in the frontals on 
either side of the internal margins of the maxillae. 
The mesethmoid does not rise to the level of the premaxillae. It 
forms a thick partition separating the nasal passages superiorly, fills 
in the frontal fontanelle, provides support for the vertex of the skull, 
and underlies the nasals. Ventrally, the mesethmoid sheaths or 
forms a thin veneer of bone around the dorsal and lateral faces of 
the presphenoid, thus lining the internal walls of the nasal passages 
superiorly. The pterygoids, apparently, send thin laminae of bone 
dorsally to sheath the external walls of these passages. The presence 
of asecond pair of passages which open at the posterior end of the 
mesethmoid gutter and lead into the brain case is a matter of unusual 
interest. These formina owe their distinctness to that portion of 
