1076 DR. R. BROOM ON 
. may be grouped together as reptiles with two temporal arches or 
modifications of the same type. In Sphenodon we have a simple 
modification of the type; in the Crocodile another, and in the 
Lizards a third where the lower arch is lost. Among extinct 
forms we have many other modifications. The Phytosaurs and 
Pseudosuchians afford the best known Triassic types, and other 
varieties of the type are seen in the Gnathodonts and the Dino- 
saurs. Youngina represents a type more primitive than any 
previously known, and one which is especially important in that 
it is very near to the ancestral form. 
The Pseudosuchian, Huparkeria, which I recently described 
from the South African Upper Triassic beds, bears considerable 
resemblance to Youngina but is very manifestly a much later 
type. It has lost the large pineal foramen seen in Youngina, 
and the post-temporal region differs considerably through the 
loss of the tabular, the reduction in size of the squamosal, and 
the increased development of the quadratojugal and quadrate. 
Yet such a type as Yuparkeria might readily be descended from 
a form like Youngina. 
Euparkeria further represents a type from which the Thero- 
podous Dinosaurs might be derived, but is too far advanced to 
have been ancestral either to the Sauropoda or Predentata. 
Youngina, on the other hand, retains the characters that we 
require in the ancestor at least of the Sauropoda, and possibly . 
also of the Predentata. 
But the most interesting point in the structure of Youngina is 
the light it throws on the origin of lizards. 
No point in reptilian structure has given rise to so many 
different opinions as the nature of the post-temporal region’ in 
lizards. In Sphenodon there is no difficulty. There is low down 
an undoubted quadratojugal, and between this and the parietal 
a single large bone which is unquestionably the squamosal. In 
the typical lizards, on the other hand, between the top of the 
quadrate and the parietal are two small bones, and the difficulty 
is to determine which is the squamosal. The upper and inner 
bone has been regarded as the squamosal by Gegenbaur, Baur, 
Gaupp, Case, and Watson; the lower and outer by Parker, 
Huxley, Cope, Boulenger, and Williston. Until a year ago I 
favoured the view of Baur, but the study of the Mosasaur skulls 
in the American Museum led me to adopt the view of Williston, 
that the outer bone is the squamosal and the inner the tabular. 
About a dozen years ago I made a study of the development 
of the pterygo-quadrate bar in a number of lizard types, and 
found that the lower end of the quadrate is fixed to the lower 
end of the epipterygoid by a small bar of cartilage almost exactly 
as in Sphenodon. So strikingly similar is the condition that it 
seems extremely probable that there was in the ancestral lizard 
a lower bar as in Sphenodon. 
While Youngina is certainly not a lizard, it throws very definite 
