28 FIELD CoLUMBIAN MusEUM—GEOLOGY, VOL, II. 
as a narrow groove to the inner posterior margin of the surface. 
Posteriorly the slit for the eighth nerve seems to be a little above 
and back of the vagal opening, in the interstice between the exoccip- 
ital and petrosal. The large cavity of this bone looks backward to 
communicate broadly with a similar cavity in the petrosal on the inner 
side. On the outer side there is a small foramen, nearly or “quite 
separated from the inner opening, also communicating with a small 
foramen in the opposed sutural surface of the petrosal. Externally 
the exoparoccipital shows a narrow fossa below the process, into 
which open the vagal and hypoglossal foramina. Above, the gently 
convex surface continues into the similar surface on the sides of the 
supraoccipitals. The posterior borders of the exoccipital and supra- 
occipital meet in an obtuse angle, which is excavated, as already 
described for hgamentous attachment. | 
The supraoccipitals are not only parial, but they are widely 
separated from each other, approaching each other only at the upper 
extremity posteriorly. They enclose between their smooth, narrow 
edges posteriorly a large vacuity, continuing the foramen magnum 
quite to the parietal roof. This relation of these bones I can not 
find paralleled in any reptiles. Though paired in the Stegocephaha, 
as also in Pariotichus, they meet in a median suture. Whether this 
peculiar structure obtains in all other plesiosaurs I can not say, 
inasmuch as the only references to the supraoccipitals which | find 
in the literature is a brief one by Andrews* concerning the bone in 
the young of Cryptoclidus, in which nothing is said of a similar 
structure, and a notice by Owent+, who describes the supraoccipital 
in Plesiosaurus dolichodeirus as a single, arched bone. | 
The inferior articular surface for union with the exoccipital is flat 
and triangular in shape, looking downward or slightly backward. It 
is pierced near its middle by the foramen for the superior semicircular 
canal. The sutural surface for union with the petrosal meets the 
exoccipital at an angle of about one hundred degrees, and is flattened 
or gently concave, and shorter than the other sutural surface. The 
external surface is moderately convex, and a little roughened. The 
posterior border is thin and smooth, deeply concave and sinuous, the 
upper extremity curved inward. The inner surface is quite smooth, 
gently convex from before backward, nearly straight to its upper 
third, where it bends strongly inward. The posterior border is short, 
thick, convex from side to side, and concave on its upper part before 
joining the sutural surface. The sutural surface above, for union with 
* Geol. Mag. 1895, p. 242. 
+ Fossil Rept. Liassic Formation, p. 8. 
