404 PROF. HUXLEY ON THE REPRESENTATIVES OF [May 27, 
in one matter of special homology it must be altered, and for gua- 
ratum=incus we must read quadratum=malleus. 
The modification of Reichert’s view which I now propose completely 
removes a difficulty which has often presented itself to my own mind, 
and which is urged with much force by Prof. Peters, in his first two 
papers. If the ércus be the homologue of the quadratum, why does 
it become so small and insignificant in the Marsupials and Mono- 
tremes, which, in so many respects, approach the Sauropsida? This 
question I have always felt could only be met by another. Why, if 
the ossicula auditis of the mammal do not represent bones of much 
more importance in the Sawropsida, does the malleus attain such a 
vast size in the Monotremata? 
If the malleus is, as I have endeavoured to prove it to be, the ho- 
mologue of the quadratum, the last question receives an obvious 
answer ; and no less readily is it intelligible why the zxeus should be 
reduced, seeing that the suprastapedial is always insignificant in size, 
and may even become a mere fibrous band, in the Sawropsida. 
The left tympanic cavity and the adjacent parts in Hehidna setosa. Some por- 
tions of the squamosal and of the periotic bones, with the tympanic mem- 
brane, have been removed. 
Mn. Mandible. Sg. Squamosal. P#.g. The pterygoid. S¢.4. The stylo-hyal. 
VIL. The portio dura. St. The stapes. Z. The incus, the greater part of the 
contour of which is supposed to be seen through the malleus (M). Pg. 
Folian process of the malleus. 77.7. The tensor tympani. 
I may mention, incidentally, that Hehidna presents other anomalies 
than those which have been described by Prof. Peters and others. 
The muscle which plays the part of the tensor tympani is very 
large and arises from the base of the skull, where it forms the roof 
of that posterior and inner region of the tympanic cavity which is 
bounded below by the pterygoid. The strong tendon of this muscle 
