SOME CARNIVOROUS THERAPSIDS. 1027 
edge of the lateral process of the exoccipital, the opisthotic, and 
the basiocecipital. 
The front face of the paroccipital process is formed by the 
pro-otic, and its lower surface is channelled. ‘The fenestra ovalis 
is a large, irregular hole, not bounded by bone in front, lying 
below the level of the base of the basioccipital condyle. The 
special process of the opisthotic which forms its posterior border 
has a wide, smooth face looking forwards and outwards, and is 
supported on the inner side by a corresponding process from the 
basioccipital. 
The basisphenoid, as shown by Case’s figures, has large tuber: 
reaching back towards the fenestra ovalis, and powerful basi- 
pterygoid processes of a more or less ordinary character. 
The basicranial and otic region of the very primitive Gorgon- 
opsid Arctops closely resembles that of Dimetrodon. It has a 
large, round basioccipital condyle and a thick basioceipital which 
supports the exoccipitals and opisthotics. The paroccipital pro- 
cesses are very powerful, lying below the small post-temporal fosse. 
The exoccipitals are extremely similar to those of Dimetrodon, 
lying on each side of the foramen magnum and sending a special 
process out towards the post-temporal fossee. The exoccipital 
even in this type is only incompletely included in the occiput. 
The foramen jugulare is high up on the back of the skull. 
The front face of the paroccipital process is formed by the 
pro-otic, and its lower border and front face are channelled by 
a groove at the inner end of which lies the large and rather 
irregular fenestra ovalis. This is placed at about the level of the 
lower surface of the basioccipital condyle, and is completely 
enclosed by bone. 
The basisphenoid forms two very large tubera, between which 
is a deep depression terminated abruptly in front by the junction 
of ridges from the tubera to form a very deep median keel con- 
tinuous with the parasphenoid. 
The tubera basisphenoidalia are far back, and end in a ridge 
just in front of the fenestra ovalis. 
The basipterygoid processes are curious horizontal plates 
standing out from the sides of the flat, vertical plate which 
forms the whole anterior part of the basis cranii. 
Scymnognathus whaitsi* differs far more from Aretops than 
the latter does from Dimetrodon in this region. 
The basioccipital condyle is no longer round, but forms a mere 
lip round the lower border of the foramen magnum. The whole 
arrangement is so remarkable that if I had not myself removed 
the thin skin of matrix surrounding it, I should not have believed 
it possible. This lip is no doubt partly formed by the exoccipital, 
as it extends up round the sides of the foramen magnum. 
* This is the specimen the lower jaw of which I described under the name 
Seymnosuchus whaitsi, Ann. & Mag. Nat. Hist. ser. 8, vol. x. p. 578, fig. 3. Iam 
not certain of the identification. 
69* 
