142 
PROFESSORS T. W. BRIDGE AXD A. C. HADDON 
mesiallv, however, the septum becomes inclined obliquely forwards, and at the same 
time gradually narrows so as to form a thick triangular band of fibres, which extends 
as far forwards as the middle of the ventral wall of the anterior chamber, and finally 
becomes continuous therewith at its apex, as well as along its converging lateral 
margins. The two lateral compartments are continued forwards to the inclined 
median portion of the septum as far as the middle of the floor of the anterior chamber, 
in the form of two gradually contracting cavities, but remain separated from each 
other b}’- a corresponding extension of the longitudinal septum along the posterior 
face of the inclined septum. Hence, as in some species of Arms, the anterior 
chamber is partially overlajiped on its ventral side by the anterior extremities of 
both lateral compartments. A single, but extremely stout, secondary transverse 
septum (t.s/) incompletely subdivides the cavity of each lateral chamber. 
The lateral and posterior margins of the bladder are curiously compressed, and 
form, as it were, a solid flattened rim with a crenulated outline, partially encircling 
the organ. With the exception of the anterior wall, the walls of the bladder are, 
])erhaps, thicker than in any other normal Siluroid, and so densely are their com- 
ponent fibres interlaced and matted together, that it is only in the dorsal wall, and, 
to a sliofht extent in the lateral walls of the anterior chamber, from whence the fibres 
which are ultimately inserted into the tripodes are derived, that any definite arrange- 
ment of them can be made out. The outer walls of both the anterior and lateral 
compartments are also greatly thickened internally by the aggregation of the fibres 
composing the inner stratum of the tunica externa into stout, vertically-arranged 
bundles, separated from one another b}" intervening slit-like sacculi. By the 
formation of these bundles internally, and the compression of the peripheral margins 
of the bladder, the lateral walls attain a thickness of nearly 4 ’5 mm. The secondary 
transverse septa, and the posterior face of the primary transverse septum, are 
thickened at their dorsal and ventral margins into ramifying root-like bundles of 
fibres, which fray out into the dorsal and ventral walls of the bladder, and finally 
blend therewith. 
Relatively to the size of the fish, the apparent size of the air-bladder, when seen 
externally, is not less than in the various species of Arius, but the actual capacity of 
the internal cavities, and especially of the lateral compartments, is really very much 
smaller. This is partly due to the extraordinary thickness of its walls, and to the 
stoutness of the different internal septa, but also in a great measure to the flattened 
condition of the entire organ. 
The comparatively small anterior chamber has its antero-lateral angles prolonged 
outwards into two small csecal diverticula, which curve inwards in front of, and in 
contact with, the adjacent portion of the anterior wall. Each caecum (al.c.) is 
deeply sacculated along its anterior margin ; but otherwise its cavity is simple and 
undivided, and freely communicates with that of the anterior chamber. 
The skeletal relations and attachments of the walls of the anterior chamber, as 
