i6 
RICHARDS. 
forward extremities in the engraving: — one of them, the 
middle one of the three, about an inch below its upper left 
hand corner; another, the lowest of the three, just a 
little above b on the margin. The septa dividing these 
cells run at right angles to the skull surface. The length 
of the upper and middle cells, measured in an anteroposte- 
rior direction, is 45 millimeters, that of the lower one is 
30 millimeters. The vertical height of all three is about 2 
centimeters ; the extent of the middle one in a horizontal 
direction — at right angles with the mesial plane of the 
skull — is about the same, viz., 2 centimeters; that of 
lower one is fully 45 millimeters. It is with the system of 
cells contained within this mastoid prominence of the 
exoccipital bone that communication from the secondary 
tympanic cavity is established through the medium of the 
"two triangular openings with smoothly rounded edges" 
mentioned by Dr. Buck as existing at the inner upper and 
back corner of this cavity ; and my further dissection has 
therefore shown that neither of these openings leads 
into the system of cells (belonging to the squamosal) 
"which surround the external auditory canal." What 
appears in the illustration to be a fourth cell lying imme- 
diately to the right of, /. c, anterior to, the extremity of 
the middle one of the three just described is in reality a cor- 
ner — the upper outer and back corner — of the secondary 
tympanic cavity cut across and opened by the section of 
the bone. It is precisely at this point that the meeting 
takes place between the two suture lines already mentioned, 
— the occipito-squamoso-periotic (or occipito-tympanic?) 
and the periotico-squamosal (or periotico-tympanic?) . 
I turn now to a brief discussion of my third illustration 
(Fig. 3). The point of view in this picture is diametrically 
opposite to that of Fig. 2 ; the observer has before him the 
inner and vertically cut surface of the entire specimen and 
is looking directly outwards, in a direction parallel with the 
