780 MR. D. M. S. WATSON ON 
Cotylosaurian family of the Captorhinide, so much so that a 
blood-relationship between them seemed most probable. 
Prof. Williston had still earlier expressed a similar opinion :— 
“That we have in Labidosaurus and its allies a persistence of 
those generalized characters which gave origin to the peculiar 
specializations of the Pelycosaurs.” One of the peculiarities of 
these Captorhinids, in which they differ from all other known 
Cotylosaurs, is the fact that they have a stapes whose distal end 
is articulated with the quadrate. We have therefore some 
justification for assuming that this peculiar condition in the 
Therapsids has not arisen within that group, but has descended 
to it from its ancestors. 
I have shown that there are very strong reasons for believing 
that the reptiles have arisen from the embclomerous Stegocephalia, 
which are the only group of Tetrapods known in Lower Carboni- 
ferous rocks. In figure A, in the plate of my paper on ‘The 
larger Coal-Measure Amphibia,” Manchester Memoirs, vol. lvii. 
pt. i., there is shown a very definite depression on the upper 
surface of the quadrate and pterygoid. ‘This definite pit occurs 
in all embolomerous Stegocephalia in which I have examined 
this region, and is obviously of importance. The only explanation 
I can find for it is that it received the outer end of the stapes. 
It is important to note that in the uncrushed skull it must have 
looked towards the auditory region. There is thus a suggestion 
of evidence that the Cotylosaurs of the family which gave rise to 
the Therapsids and the Captorhinids received the condition of 
having the distal end of the stapes articulated with the quadrate 
from their amphibian ancestors, as they in turn passed it on to 
their descendauts. 
There are strong reasons for believing that the Stegocephalia 
have arisen from the Rhipidistian group of the Crossopterygian 
fishes (the Osteolepids), which were no doubt technically, though 
probably only to a slight extent, functionally hyostylic. In these 
fishes the hyomandibular, which has been seen only by Dr. Traquair 
in Rhizodopsis (an observation I have been unable to check from 
the material in the British and Manchester Museums, which 
includes at least the greater part of that which was before 
Dr. Traquair), no doubt articulated with the otic region of the 
cranium and with the quadrate, as in all fishes. There are many 
reasons for believing that the stapes of a Tetrapod is homologous 
with the hyomandibular of a fish, and it is probable that in the 
embolomerous Stegocephalia, which had just arisen from fishes, the 
primitive connection between the distal end of the hyomandibular 
or stapes and the quadrate was retained. In later Amphibia 
(Eryops, Trimerorachis, Cyclotosaurus, to mention only types in 
which it is known in place) it lost this connection, and its distal 
end is connected with a tympanic membrane stretched across the 
otic notch. 
It thus seems to be fairly probable that the connection between 
the distal end of the stapes and the incus in a mammal has been 
