178 MR. D. M. 8. WATSON ON THE 
Comparison of these short descriptions will show that the only 
features which are of any importance * in which the five great 
groups agree are :— 
1. There is one lateral temporal vacuity. 
‘The material suggests that in primitive types this 
was bounded by the postorbital and squamosal alone, 
and is hence not homologous with either of those of 
Sphenodon. | 
. There is only a squamosal in the temporal region. 
. The occiput is plate-like, the supraoccipital being broadened 
to a wide flat plate, which separates very widely the small 
posttemporal vacuities. 
4, There is a single interparietal formed by a fused pair of post- 
parietals, which, with the tabularia which are usually 
present, is placed entirely on the back of the skull and 
overlaps the supraoccipital. 
5. The brain-cavity is very high. 
6. The opening from the brain-cavity to the ear is very low 
down. 
7. The stapes articulates with the quadrate. 
8. The angular is flat and notched. 
9. The interclavicle is always flat and wide, not T-shaped. 
0 
1 
ts bo 
. There are two coracoidal elements. 
. The anterior coracoidal element does not contribute to the 
glenoid cavity, 
12. There are always two centralia and five distal carpals when 
the carpus is well ossified. 
13. There are always two proximal and four distal tarsals and 
one centrale when the tarsus is well ossified. 
Of these characters, which include all common to all South- 
African Therapsids that are likely to be of taxonomic importance, 
Nos. 9 & 10 are merely primitive features, and so in all probability 
are 12 & 13. 
In fact, the characteristic features of the Therapsids, which 
show the real individuality of the group, are those numbered 1-8. 
Numbers 3-6 are really so connected as to be essentially one 
character, and with the condition of the angular and stapes 
are the only features which we could hope to recognise in a 
Cotylosaurian ancestor. 
It will be found that all these characters, except the exclusion 
of the anterior coracoidal element from the glenoid cavity, and 
the occasional presence of a vestigial supratemporal in the 
temporal region, occur in the various American types which have 
been included in the Pelycosauria. , 
Dimetrodon, for instance, has a single temporal vacuity, its 
* They, of course, agree in such characters as the presence of parietals, prefrontals, 
lachrymals, ctc. 
