142 Cincinnati Society of Natural History. 
which was doubtless so where the borders were more nearly com- 
plete), is here more lenticular, being less than half an inch laterally 
and one and a half inches antero-posteriorly. The section of the 
same is diagonal from before backward and downward. ‘The 
lower opening narrows rapidly forward until the right and left 
walls are scarcely more than a line apart. The upper channel has 
much the greater capacity for admitting air. 
The greatest distance between the upper and lower channels, as 
shown here, is about three-fourths of an inch. At the point where 
the upper and lower openings come together, as they extend for- 
ward, there is a horizontal, top-shaped aperture in the bony parti- 
tion separating the nostrils. This opening has the vertex forward, 
and is about one inch by five-eighths of ar inch. It will be noted, 
then, that the air from either nostril can pass backward to the right 
or left, and both above and below. 
The point where the double posterior nares joins the anterior is 
between and immediately above the bases of the incisors. The 
broad nasals and the wide area above the imbedded arches of the 
incisors and between the elevated margins of the premaxillary, 
give a good idea of the extent of the ethmoid region. 
The right nasal bone is three and one-half inches long (with 
about a half inch broken off) by one and one-fourth inches broad. 
In Castor, Fiber and Arctomys the widest portion of the nasals is at 
or near the front, while here the widest portion is posterior to the 
middle. 
The dentition is perfectly represented, for, though two grinders 
are lost from the right upper side and one from the left lower, yet 
the other sides are complete and perfect. 
The dentition in general is superior to that of the Clyde skull 
as seen in Ward’s cast. Especially is this seen in the upper 
incisors, the entire length of which, following the outer curve, is 
nine and one-half inches. They project from their sockets, meas- 
ured on the front side, fully three and one-half inches. Posteriorly — 
they are strongly braced and buttressed by a forward and down- 
ward projection of bony casing, which not only supports them on 
the lingual side, but thrusts in between and extends downward 
nearly to the backward border of the beveled crowns. ; 
The same is seen in Castor, but it is even relatively greater in 
Castoroides. Inthe Clyde skull, the teeth, where they leave the 
sockets, are thrown half an inch apart by this interdental plate of 


