250 Mr. C. W. Andrews on the Structure 
nasal passage. In a subsequent paper on the skull of Plio- 
saurus grandis from the Kimmeridge Clay, Owen* again 
refers to these openings as palato-nares, and describes the 
true internal nares as nervo-vascular foramina. 
The postero-external angle of the hinder portion of the 
pterygoids is prolonged outwards and backwards into the 
strong bar which unites at its distal end with the quadrate, 
as described above. 
On the palatal surface of the posterior ramus a high crest 
commencing near the hinder end of the posterior median 
suture runs outward and forward in a curve; externally it 
passes into the thickened hinder edge of the lateral ramus. 
This crest is shown in Pl. XIII. fig. 2, where it is marked 0. 
In the hinder part of its course in specimen 4 it is 2°5 centim. 
high. 
On the dorsal surface of the pterygoid immediately external 
to the posterior palatine vacuities may be seen in specimen 1 
the base of the columella cranwi (epipterygoid): this region 
being much crushed in all the specimens, no detailed descrip- 
tion of this bone is possible; it can, however, be seen that 
its upper extremity closely united with the lower edge of the 
parietal (Pl. XIII. fig. 1, col.). In a skull of Pliosuurus 
in the British Museum this structure is better preserved, and 
will be described on a future occasion. 
Before attempting to determine the structure of the temporal 
arch and of the dorsal surface of the skull the bones of the 
basis cranti may be noticed. 
The bastoccipital is a very massive structure ; it bears the 
whole of the occipital condyle, which is very convex and 
slightly oval in outline, its longest diameter being transverse 
and measuring 4 centim. ‘The lateral regions of the ventral 
surface of the bone are prolonged outward into a pair of very 
stout processes, the truncated ends of which look outward and 
downward and are very rugose ; they probably served for the 
attachment of muscles, but their anterior portion may have 
been covered by the pterygoids. The upper surface bears 
two roughly triangular slightly depressed areas for union 
with the exoccipital. The interval between these surfaces 
is very small, the basioccipital having taken but a small 
share in forming the actual margin of the foramen magnum. 
The exoccipitals which formed the lateral borders of this 
opening are massive bones, but in all the available specimens 
are too crushed for description. It can, however, be seen 
that they—or, rather, the opisthotic, which is no doubt fused 
* “Fossil Reptilia from the Kimmeridge Clay,” part 3 (Mon. Pal. 
Soc. 1869) p. 4. 
