744 MR. D. M. S. WATSON ON 
sexes in living Lizards, and I think it is extremely probable that 
the same reason accounts for the difference between my two 
skeletons. 
Affinities of Procolophon. 
That Procolophon is a Cotylosaurian has been recognised by all 
recent workers. Its right to inclusion in this group depends 
on its roofed skull (it is perfectly obvious from the examination 
of well-preserved skulls that the large opening on either side is 
entirely an enlarged orbit and not, as has sometimes been sug- 
gested, a joint orbit and temporal fossa), and on the typically 
Cotylosaurian vertebre which are very similar to those of 
Labidosaurus. 
Mere reference of an animal to the Cotylosauria means very 
little, as that great group includes animals which differ at. least 
as much from one another as do a Lizard and Sphenodon, or even 
a Tortoise and a Crocodile. 
When discussing Pariasaurus recently I divided all Cotylo- 
sauria, after the removal of the very primitive Seymouria, into 
two groups, from the way in which they obtained a vertically 
standing quadrate, one process leading to the obliteration of the 
otic notch, the other to its exaggeration. 
To the first group belong the Captorhinids, Pariotichids, and 
Pantylids; to the second, Diadectids and Pariasaurids. 
Procolophon has an enormous otic notch, and of course falls 
into the second group. 
Procolophon and its allies are the latest of all Cotylosauria, 
occurring, so far as is definitely known, only in the Trias after 
the disappearance of all other types. In consideration of their 
age we should expect to find many advances in their structure. 
[Any study of paleontology shows that all allied animals tend, 
with mere passage of time, to change their structure in certain 
definite ways which are common to whole groups and occur 
independently in different lines of descent. These changes, 
which are usually independent of the adaptations of the animals 
in which they occur, take place at different rates in the distinct 
lines, and the sets of changes of different parts of the same 
animal are independent and may proceed at very different rates. 
Changes of this type I refer to as advances; they are quite 
different from specialisations, which themselves are of two kinds, 
adaptations to some special mode of life, and what are usually 
regarded as specific characters, 7.e. such differences between 
animals as form the typical Mendelian allelomorphs; it is very 
probable that these characters once established, perhaps as salta- 
tions, are very stable, and may persist in a recognisable form 
during a long series of advances. | 
The advances in the structure of Procolophon are :— 
1. The rounded occipital condyle. 
2. The vertical quadrate. 
