Nye MR. D. M. S. WATSON ON THE 
Seymouria has retained the structure of the most primitive 
reptiles. 
If we consider later large Amphibia, we find that there isa 
tendency, which is repeatedly expressed, to replace the inclined 
quadrate by a vertical one; this is the case for example in Butra- 
chosuchus, Brachyops, Plagiosternwm, ete. 
Exactly the same change takes place in Cotylosaurs. There 
are two extreme ways in which an inclined quadrate can be 
converted into a vertical one, either (a) the lower end is kept 
fixed and the upper end swung backwards, or (0) the reverse takes 
place. 
The process (a) will result in a complete obliteration of the otic 
notch, and the squamosal will be brought into line with the 
tabular on the extreme back of the skull; this is probably the 
type of change which has produced such types as Labidosaurus 
and Captorhinus. In such types the tabulare, if it be present, 
being firmly fixed between the squamosal and the postparietal, 
does not really require any additional support, and the outer end 
of the opisthotic is free to wander down to the region of the 
quadrate condyle to render support to that bone. 
The process (4) results in the retention of the otic nee and in 
fact in its exaggeration. ‘To it we owe types like Diadectes and 
Procolophon, with an enormous otic notch overhung at the top 
by the squamosal and tabulare, and with the whole of the quadrate 
in advance of the basioccipital condyle. In these types the 
opisthotic is far removed from the quadrate and has no possible 
opportunity of supporting it. 
From such a type Pariaswurus was undoubtedly derived by 
the subsequent obliteration of the otic notch, by the develop- 
ment of secondary plates from the squamosal and quadrato- 
jugal behind the quadrate. Even in these types the opisthotics 
have a tendency to rotate downwards, probably to extend the 
area for the attachment of neck muscles; in doing so they take 
the tabulars which are attached to their outer ends with them. 
Another important type of change has been pointed out by 
v. Huene. This is that the postparietals and tabulares, which in 
Stegocephalia and Seymowria ave bones on the upper surface of 
the skull, tend in later Cotylosaurs to be reduced to thin films 
placed vertically on the back of the skull. This change is a very 
important one. 
We may now consider the relationship of Pariasawrus to 
such other Cotylosaurs as are sufficiently well known to make a 
comparison of any value. 
Seymouria bayloriensis differs in the following characters, which 
are primitive ones :— 
(1) The shape of the skull. 
(2) The narrow otic notch. 
(3) The inclined quadrate. 
(4) The upwardly directed opisthotiec. 
