134 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
stratum to the outer side of each trIpus, with which, however, they are in no way 
directly connected (fig. 48, in.st.). Traced from the anterior into the antero-lateral 
and lateral walls the fibres of the same stratum, are still vertical, but in the dorsal 
w’all become transverse, or at all events oblique, and nearly, if not quite, coincident 
in direction with the fibres of the outer stratum ; nevertheless, none of these fibres 
appear to reach the tripodes, or become directly attached thereto, but thin away 
before reaching the crescentic processes of those ossicles. The inner stratum is 
relatively thin, and hence the triangular sheets which form the greater part of the 
dorsal wall of the anterior chamber are mainly composed of the converging fibres of 
the outer stratum. We have already pointed out that the subvertebral process is so 
closely applied to the anterior wall of the bladder as to produce in it a deep median 
groove, and we may add that the distal extremity of the process is attached thereto 
by fibrous tissue. 
From a comparison of Arms and Macrones it will be obvious that the extent to 
which the anterior wall of the air-bladder is attached to the axial skeleton varies 
greatly in different Siluroids. In most of the genera with normal air-bladders that we 
have so far described, as for example in Macrones, the whole thickness of the mesial 
portion of the wall is attached dorsally to the lateral surfaces of the complex centrum, 
to the radial nodules, and to the ventral ridges of the tripodes, but laterally to this 
point, on each side, the inner and outer strata of the tunica externa behave dif- 
ferently in different genera. In Macrones and its allies both strata are continued 
into the dorsal wall, and eventually become inserted into the anterior part of the 
convexity of the crescentic process of each tripus. In Arms, on the contrary, the 
outer stratum in this region becomes disassociated, as it were, both from the tripus 
and the inner stratum, and firmly inserted by its dorsal margin into the anterior edge 
of the transverse process of the fourth vertebra. Variations may also be noted in the 
extent to which the fibres of the interior chamber are inserted into the tripodes. Thus 
in Macrones, all the fibres of the inner stratum in the lateral portions of the anterior 
wall, as well as those of the same stratum in the lateral walls, are prolonged into the 
dorsal wall and eventually become inserted into the tripodes ; but in Arius the corre- 
sponding fibres, although traceable into the dorsal wall, thin away and disappear as a 
definite stratum without reaching or becoming directly attached to these ossicles. 
Hence it is that in the latter genus, the only fibres directly inserted into the 
tripodes are those which form the outer stratum of the tunica externa of the 
lateral walls and are subsequently prolonged into the dorsal wall in the form of 
stout triangular sheets, whereas in the former genus the fibres of both strata in 
the anterior as well as in the lateral walls, with the exception of the skeletally 
attached mesial part of the anterior wall, converge in the dorsal wall to the tripodes 
and are directly attached thereto. With reference to these variations it may be 
pointed out that Platystoma and other normal Pimelodinae more closely approach 
the Arioid than the Macrones type in the extent and nature of the skeletal 
