8 
RICHARDS. 
little to its formation. As regards the periotic bone, I 
have spoken above of the relatively small pneumatic cells 
found in its posterior portion and have said that this was 
probably the true mastoid portion mentioned by Flower. 
Kostlin however denies the existence of any mastoid 
portion (pars mastoidea) in the elephant; and without 
a violent disruption of the specimen now under discus- 
sion I can not determine whether these pneumatic cells 
belong strictly speaking to the periotic bone or to the 
tympanic, with which latter the periotic is even in the 
young elephant completely united (see Flower, op. cit., 
p. 181). I can detect no line of separating suture between 
the cell-containing lower posterior part of what certainly 
seems an integral portion of the dense periotic and the 
higher-lying and more forward-lying part of the periotic, 
containing the vestibule, cochlea, and semi-circular canals 
but otherwise quite lacking in cellular structure. 1 As 
to these pneumatic cells which we may pretty safely 
assume to be within the periotic bone, it would perhaps be 
more correct to speak of them as a single cell with freely 
communicating and wide-mouthed side crypts than to re- 
gard each of these crypts as a separate cell ; this single cell 
with irregular side crypts or alcoves is as yet the only 
pneumatic space I have been able to detect within the per- 
j . 
'I have said that the cells within the mastoid process of the exoccipi- 
tal hone do not communicate with any cells within the periotic. I may 
add that in the line of this suture and between these two bones is found a 
small portion of a third bone independent of both the others and having 
within it small cellular spaces communicating with neither of these two. 
To what bone this portion belongs I do not know ; clearly not to the 
exoccipital; seemingly also not to the periotic: for the upper part of the 
periotic is separated from it by a line of suture. It may be a portion 
of the tympanic (although the latter bone is firmly joined to the periotic 
below) ; or it may belong to the scmamosal. I am disposed to regard it 
as a portion of or process from the latter. This process, to whichever 
of these two bones it may belong, takes part in the formation of the outer 
wall of the upper part of the secondary tympanic cavity described by Dr. 
Buck. 
