202 REPORT—1846. 
6 
upper and forepart of the tympanic cavity so exposed. So much for the 
facts of the argument*. 
We may next ask, Is the formation of the upper boundary of the meatus 
externus an essential character of the squamosal in mammals; or is it not 
rather a secondary consequence of the expansion and application of that bone 
to the side of the cranium in this particular class? If we were desirous of 
obtaining a homological character by comparison of the contour of the 
meatus externus or the tympanic cavity in mammals and birds, ought we 
not rather to select the lowest and most ornithoid of mammals, as best cal- 
culated to throw light upon the real nature of the modifications of this part 
of the skull in the respective classes? In the echidna, then, we find that 
the squamosal does not form the whole of the superior border of the shallow 
tympanic cavity, but that the mastoid forms the posterior half of that border, 
and sends a short obtuse process downwards (at 16, fig. 12), which overhangs 
the cavity and gives attachment to the tympanic (2s). Behind the mastoid 
is the exoccipital. Now in birds the antero-posterior extent of the cranium 
between the exoccipital and post-frontal bones is much shortened as compared 
with mammals, and this modification I interpret as the result, in a great de- 
gree, of the entire removal of the squamosal from the cranial parietes. Of 
the homology of no. 4 as a part of the exoccipital there has been no question, 
although its development, and the share it takes in the lateral parietes of the 
head, is increased, as compared with most mammals, rather than diminished. 
The exoccipital constantly unites anteriorly with the mastoid in mammals, 
from man down to the echidna; but the extension of the squamosal back- 
wards to articulate with the exoccipital is far from being a constant character 
in mammals. We ought on that ground therefore to conclude that the bone s, 
which articulates with the fore-part of the exoccipital in the bird, is the 
‘ mastoid,’ rather than that it is the ‘squamosal.’ It overhangs the tympanic 
cavity by a longer or shorter process ; but being more advanced in position, 
partly by the development of the exoccipital behind, and the non-interposition 
of a squamosal between it and the alisphenoid in front, it overarches the 
middle of the upper instead of the posterior part of the upper border of the 
tympanic cavity in the bird; but it is still in great part posterior to the tym- 
panic pedicle, a relative position which is foreign to the squamosal. The 
process of no. s resembles the mastoid process in mammalia, inasmuch as 
it terminates freely in most birds; and in those, the parrot for example, in 
which it joins another process to form a zygoma or bridge over the temporal 
fossa, that process answers to the post-frontal, the very bone which the mas- 
toid similarly joins in the crocodile, and does not answer to the malar bone, 
which the squamosal joins in both mammals and crocodiles. 
The mastoid always coalesces with the petrosal, rarely with the squa- 
mosal, in the mammalia; such coalescence is therefore a more constant cha- 
racter of the mastoid than of the squamosal, and the argument becomes 
cumulative in favour of the mastoid or petromastoid character of no. s in the 
bird. When we remove the squamosal in the sheep we bring away the man- 
dible which articulates with it, but we leave the distinct and independent tym- 
panic closely articulated to the petromastoid. Precisely the same thing 
happens in the rodentia, in the marsupialia, and especially in the echidna, 
in which the tympanic has the slightest connection with the squamosal. The 
articulation of the tympanic therefore with the petromastoid is a more con- 
stant character than its articulation with the squamosal ; therefore the arti- 
culation of the unquestioned tympanic bone in birds with the bone no. s is a 
* The same formation of the upper boundary of the meatus externus is shown by Geoffroy 
in the young fowl.—Annales du Muséum, x. pl. 27. fig. 2. V.Q. 
—— 
