86 MEMOIRS OF THE CARNEGIE MUSEUM 
In Ichthyosaurus the lower articular end is shown as only articulating with two 
bones, the basioccipital and exoccipital, the stronger surface being opposed to the 
latter. In B. discus, as has been indicated above, this bone articulates with three 
elements the longest face being opposed to the basioccipital. 
Woodward” shows the opisthotic of I. longifrons as articulating exclusively with 
the exoccipital and instead of extending outward and upward, it is inclined outward 
and slightly downward reaching the inner border of the quadrate. In his diagram 
the stapes are absent. Frass” gives a posterior view of the skull of I. qwadrissicus in 
which the opisthotic articulates medially with the exoccipital and slightly if at all 
with the basioccipital. Outwardly the upper end is free. In this species the parietal 
is interposed between the free end of the opisthotic and the squamosal. Cope’s” 
diagram of this view of the skull of Ichthyosawrus approaches the form under con- 
sideration more nearly than any brought under the writer’s observation. Though 
Cope was probably mistaken in the placing of the stapes as not articulating with 
the basioccipital. 
MEASUREMENTS. 
No. 878. Greatest length of opisthotic......................cccceceeceeceeceee eee eeees 60 mm. 
SO iis us breadth distally of opisthotic.....................c:eceepeeeeeee 40‘ 
‘© 878. es e proximally of Opisthotichesssscese-sceeeseseeeeneaes Oy pt 
Stapes (st.).—The stapes are two trihedral bars that extend downward and outward 
from the lateral inferior borders of the basioccipital. The superior or articulating 
end is the most expanded, being divided into two distinct articular faces. The up- 
per face is opposed to a similar articular surface on the opisthotic at about the mid- 
dle of the side of the basioccipital. The lower articular surface is in contact with 
the lower side of the basioccipital. The outer end of this bone is slightly expanded 
and rests in a groove on the inner lower posterior surface of the quadrate. The posi- 
tion assignedit by Baur“ in the Ichthyosauria. Seen posteriorly the surface is slightly 
rounded. Viewed from the outer end this bone is triangular in form. Above, the 
inferior bar of the squamosal comes down and passes under the superior lateral 
margin. The lower side apparently, rested upon the posterior superior surface of 
the pterygoid. The foramen shown in all posterior views of the skulls of Ichthy- 
osauria which I have studied appears to be entirely closed by the inferior branch of 
the squamosal in B. discus. Although by an examination of the different views 
referred to above, hardly any two agree in the exact shape and_ position of these 
21 Woodward, lit. cit., p. 178, fig. 111. 
22 Frass, lit. cit., Plate II., figs. 3-8. 
23 Cope, lit. cit., p. 199, fig. 2. 
* Baur, G., ‘‘On the Morphology and Origin of the Ichthyopterygia,’’ Amer. Nat., Vol. XXI., 1887, p. 837. 
