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 

 snpraoccipitals. The posterior borders of the exoccipital and supra- 

 occipital meet in an obtuse angle, which is excavated, as already 

 v described for ligamentous attachment. 



The snpraoccipitals 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 Stegocephalia, 

 as also in Pariotic/u/s, 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 I 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 Plcsiosaiirus dolicliodcints 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 w T ith 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. 



t Fossil Rept. Liassic Formation, p. 8. 



