784 MR. D. M. S. WATSON ON 
membrane, but as in all these types the squamosal never shows 
any suggestion of an auditory groove or of the attachment of a 
membrane, I think it possible that they were without one, a not 
unknown condition. 
In 1910 W. K. Gregory, from the position of the anditory 
groove in Cynognathus, inferred that the tympanic cavity and 
membrane were below the reduced quadrate and articular. From 
these relations and the fact that in the ontogeny of a mammal 
the tubo-tympanal cavity grows up round the auditory ossicles 
which arise outside it, he suggested that phylogenetically this 
upgrowing of the tubo-tympanal sac around the vestigial quadrate 
and articulare may have caused them to share in its vibrations, 
and thus to take on an incipient auditory function before their 
old suspensory function had ceased. 
This is exactly the conclusion to which the above discussion, 
founded mainly on quite different evidence, has led us. 
In mammals the centre of the tympanic membrane is placed 
in connection with the chain of auditory ossicles by the handle 
of the malleus being fastened to its middle layer. In no 
Therapsid that I have yet examined is there a process of the 
articulare which could touch the membrane, so that it Is m- 
herently probable that the manubrium is a mammalian innovation. 
Tn ontogeny it arises very late, chondrifying much later than the 
incus and body of the malleus in erameles, but being apparently 
a veal part of the latter bone. 
It thus seems impossible that the membrane of Therapsids 
should be brought into connection with the fenestra ovalis in the 
ordinary mammalian way, and as the whole of the preceding 
arguments are meaningless if they have no membrane, it seems 
certain that another connection between the stapes and the 
membrane must have existed. 
In a remarkably able and suggestive paper, Dr. Gregory has 
put forward the following explanation :— 
“That in the most primitive Cynodonts, such as Bauria, there 
was an extra-columelia, resting against a tympanic membrane 
behind the squamosal, which had been differentiated out of the 
tissue lying between the endodermal epithelium of the tympanic 
eavity and the epidermis: that with the spread of the tympanic 
cavity the differentiation of the future tympanic membrane also 
spread, until it included the stretched skin on the posterior end 
of the jaw below the quadrate and articular and above the 
angular: that concomitantly with the reduction of the quadrate 
and articular and the detachment of the angular and goniale 
from the dentary, the newly differentiated portion of the tym- 
panic membrane became functionally more active than the old 
‘reptilian’ portion : that in this way the old membrane together 
with the extra-columella became vestigial, while the new membrane 
beeame altogether free from the dentary, but remained fastened 
both to the angular, which gave rise to the tympanic bone, and 
to the retro-articular process of the articular, which gave rise te 
