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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
of longitudinally disposed fibres, sometimes thickened into stout inwardly projecting 
ridges, extends into both the anterior and posterior walls, and shares the skeletal 
attachments of the anterior and posterior pillars.* Although, as a rule, extremely 
tliin, the median portion of the dorsal wall, over an area bounded in front and behind 
by the anterior and posterior pillars, and laterally by the dorsal walls of the two 
outwardly bulging halves of the chamber, is always firmly attached to the ventral 
and lateral surfaces of the complex centrum, and possibly also to those of the fifth 
centrum. 
The attachment of the walls of the anterior chamber to movable ossicles (the 
tripodes) is effected by the convergence of the fibres of both strata of the tunica 
externa of the anterior and lateral walls into the dorsal wall in the form of two 
triangular sheets, and their ultimate insertion into the crescentic processes of the 
tripodes, which are situated near the anterior and inner corners of the lateral halves 
of the anterior chamber. As pointed out in the special case of Macrones nemurus, 
it is generally also the case in most other normal Siluroids that the fibres, which at 
one end of their course are inserted into the movable tripodes, are at the other 
extremity absolutely or relatively fixed, the fibres of the inner stratum of the tunica 
externa being successively continued from the ossicles into the dorsal, the lateral or 
antero-lateral walls, and eventually into the ventral wall, and from thence into the 
primary transverse septum to the skeletal attachments of the latter, while the 
curvilinear fibres of the outer stratum follow the same course until they reach the 
lateral walls, from whence they are ultimately traceable into the relatively fixed or 
inexpansible walls of the lateral compartments. The variations in the extent to 
which these fibres are attached to the tripodes are mainly confined to one feature. A 
slip of fibres derived from the median portion of the anterior wall is always inserted 
dorsally into the ventral ridge of each tripus, or directly into the ventral surface of 
the ossicle when the ridge fails to be developed. Laterally to this point the fibres 
forming the whole thickness of the tunica externa of the anterior and lateral walls 
may converge in the dorsal wall and become attached to the tripodes {e.g., Macrones) ; 
or as in many other Siluroids {e.g., Arius, Pimelodus, &c.), the outer stratum of the 
anterior wall is continuously attaclied by its dorsal edge to the transverse process of 
the fourth vertebra, and only the comparatively thin inner stratum, in addition to 
the fibres of both strata from the lateral walls, extend into the dorsal walls and 
constitute the triangular sheets. In the latter case but few, if any, of the fibres of 
the inner stratum reach the tripodes, which, in consequence, only receive the direct 
insertion of the outer stratum of the tunica externa of the lateral and antero-lateral 
walls. 
* A further device, with the like object of giving additional rigidity to the ventral wall, may, 
perhaps, be found in the obliquity of the primary transverse septum, and in the tendency of the lateral 
compartments to more or less completely overlap the anterior chamber on its ventral side, which are 
such obvious features in the air-bladders of many species of Arius and some Pimelodina). 
