ON THE FAUNA AND FLORA OF THE TRIAS OF THE BRITISH ISLES. 157 
The basisphenoid is exposed as a bone nearly 1 cm. wide between 
the down-turned pterygoid processes. This bone contracts behind to a 
width of about 5 mm. ; behind this it has two lateral wings, which are, 
however, badly preserved and exposed. These wings probably corre- 
spond with the two knobs which occur in Sphenodon just before the 
junction of the basisphenoid and basioccipital. The small piece of 
bone immediately behind is the basioccipital. This is 1.5 mm. long, 
and is succeeded by the atlas, which is not easily distinguished from it, 
being represented mainly by a hole. The palate, as a whole, much 
resembles that of Sphenodon, the chief differences being as follows: 
(1) There appears to be no suborbital foramen in front of the trans- 
palatine, whereas in Sphenodon there is a small one; this difference is 
not a very important one as is shown by the similar variation in 
Sauropterygians. 
(2) In the great height of the pterygoid process and attached ptery- 
goids Rhynchosaurus differs considerably from the condition in ~ 
Sphenodon. 
(3) The great distance between the two pterygoids in this region in 
Rhynchosaurus is also a conspicuous difference from the other genus. 
(4) The fact that the posterior ramus of the pterygoid is only a 
slender bar in the Triassic genus renders its appearance different from 
that of Sphenodon. 
The palate of Rhynchosaurus resembles strongly that of Hypero- 
dapedon in certain respects, whilst differing from it more widely than 
does that of Sphenodon in others. 
Apart from the dentition, the chief differences between the two 
Triassic genera are as follows : — 
(1) In Hyperodapedon the pterygoids meet in the middle line and 
appear completely to conceal the basisphenoid from a ventral view; this 
is a pronounced difference from Rhynchosaurus, in which the whole of 
the basis cranii can be seen from below. 
(2) In Hyperodapedon the pterygoids appear to be only just below 
the main ventral surface of the basisphenoid, and are possibly in actual 
contact with it, whilst in Rhynchosaurus they are carried down on two 
long processes to about 1 cm. below that level. 
In these two respects Sphenodon seems to be exactly between the 
fossil genera. On the other hand, these latter agree in the absence of 
any infraorbital foramen and of the foramen between the quadrate and 
the quadratojugal. They also agree in the very pronounced downward 
curve of the premaxille, which in the recent Sphenodon bears two teeth 
and is not itself appreciably decurved. 
On the whole of the evidence, I think that Rhynchosaurus and 
Hyperodapedon should not be placed in the same family, but that they 
should be included in a super-family, which would most probably also 
include Stenometopon, but not Sphenodon. 
Explanation of Plate IV. 
Fig, 1.—Under surface of the skull of Rhynchosaurus in the Manchester 
Museum. Nat. size. 
ju. Anterior end of the misplaced jugal. 
q- je The quadratojugal. 
post. pt. The posterior ramus of the pterygoid 
B. 8. The basisphenoid, 
