ON THE ANATOMY OF FISHES. 
133 
parallel ridges of the ventral wall as clearly owe their existence to an excessive 
thickening of the inner stratum of longitudinal fibres, which continuously extends 
between the anterior wall and the primary transverse septum and their respective 
attachments to the skeleton. The special development of these fibres, and their 
concentration into strong inwardly projecting ridges, is probably to be correlated with 
the greater thickness of the walls of the anterior chamber in Arius. Functionally, 
the ridges may be compared to a more or less rigid skeletal structure, arch-like in 
shape, and with the convexity of the arch directed towards the ventral surface, thus 
rendering the anterior, posterior, and ventral walls of the anterior chamber rigid and 
unyielding, and therefore necessarily causing any increase or diminution in the internal 
capacity of the chamber to depend mainly, and probably solely, on the movements of 
its lateral walls. The projection of these ridges into the cavity of the chamber, com- 
bined with the strong median ridge which the impression of the sub vertebral keel 
gives rise to in the dorsal wall, has the effect of reducing the communication between 
the lateral halves of the chamber to a relatively small oval foramen. (See Ketengus 
typus, fig. 50.) The walls of the compartment are perfectly smooth internally, and, 
laterally, are in contact with the cutaneous areas (fig. 48, l.c.a.). 
In the arrangement and skeletal attachments of the fibrous sheets forming the 
walls of the anterior chamber, Arius exhibits several features in which it differs from 
Macrones, and many other normal Siluroids that we have so far described. In the 
proper anterior wall the fibres forming the median portions of both the inner and outer 
strata of the tunica externa are closely coherent, and on the ventral surface of the 
anterior end of the complex centrum split into two diverging bundles, v/hich pass 
dorsally, one on each side of the median line, and ad their dorsal margins have the 
usual insertions into the lateral surfaces of the centrum, the radial nodules, and the 
ventral ridges on the crescentic processes of the tripodes. (See BatracJiocephalus 
mino, fig. 52, ap.) Externally to these skeletal attachments, on either side, 
the two strata become more easily separable, although still connected by scattered 
fibres passing from one to the other, but the fibres of the outer stratum, instead of 
extending into the dorsal wall and becoming attached to the tripodes, are inserted 
by their dorsal edges into the whole extent of the decurved anterior margins of the 
transverse processes of the fourth vertebra, including the distal extremities of the 
two processes, the insertion being posterior to the dorsal attachment of the transverse 
membrane (fig. 52, t.p.ki., o.st.). Traced from the anterior wall into either lateral 
wall the fibres of the outer stratum acquire the characteristic curvilinear disposition, 
and, if traced thence into the dorsal wall, converge in the form of thick triangular 
sheets to their ultimate insertion into the crescentic processes of the tripodes (o.st.'). 
The fibres forming the inner stratum of the tunica externa of the anterior wall on 
either side of the median skeletally attached portion have the normal vertical 
disposition, but become longitudinal, or slightly oblique, if traced thence into the 
dorsal wall, where they cross at right angles the converging fibres of the outer 
