138 
PROFESSOKS T. W. BRIDGE AND A. C. H ADDON 
expense of the lateral compartments ; or, in other words, variations in the relative size 
of the anterior chamber in comparison with the bulk of the Fish are confined within 
narrower limits than is the case with corresponding variations in the size of the lateral 
chambers. The relation of such variations, and others of a similarly minor character, 
to particular species may be briefly noted. 
In Arms sagor the subvertebral process is somewhat spatulate at its free distal 
extremity. The lateral walls of the anterior chamber are extremely thin over 
definitely circumscribed oval areas, which are in close relation externally with the 
lateral cutaneous areas. 
In Arius venosus the walls of the bladder are not only very thick, but the compo- 
nent fibres of the tunica externa are so densely interlaced and matted together that 
their course can scarcely be traced beyond the triangular sheets of the dorsal wall of 
the anterior chamber. 
In Arius ccelatus, and in one or two other species, the outer wall of each lateral 
chamber is strengthened interiially by numerous vertically-disposed rib-like aggrega- 
tions of fibres, separated by narrow intervening slits, and these may even extend 
forwards into the outer wall of the anterior chamber. 
In Arius australis the air-bladder has particularly stout and rigid walls, and the 
obliquity of the primary transverse septum, especially of its mesial portion, is so great 
that its ventral edge extends nearly as far forwards as the anterior wall of the bladder 
before coalescing with the ventral wall ; hence, the lateral compartments are prolonged 
as gradually contracting cavities on the ventral side of the inclined septum, nearly as 
far forwards as the anterior wall of the anterior chamber, but nevertheless remain 
separated from each other by a corresponding extension of the longitudinal septum. 
To a greater or less extent, this feature is characteristic of most species of Arius, but 
it is exceptionally well marked in this particular species. 
In Arius latiscutatus there is but one secondary transverse septum in each lateral 
compartment. 
Arius assimilis differs in no essential respect from Its East Indian and African 
allies. The walls of the air-bladder, however, are extremely thin. Two short rudi- 
mentary secondary transverse septa are present in each lateral compartment. 
In Cuvier and Valenciennes' great work (8) the air-bladder in several species of 
Arius is said to be provided with thick extrinsic muscles, as, for example, in Arius 
ccelatus and A. milherti. The latter species we have had no opportunity of dissecting, 
but certainly no such muscles are present in A. ccelatus, or in any other species of the 
genus that we examined. 
If Dr. Taylor’s account (38) of the air-bladder of Arius gagora be correct, this 
species must possess a bladder of a very different type from any that we have met 
with in other species of the genus. According to this observer, there are two air- 
bladders, one on each side, enclosed in an osseous cup attached by a narrow stalk to 
the body of the “ first” vertebra, close to its junction with the cranium. The mouth 
