160 
PROFESSORS T. W. BKTDGE AND A. C. HADDON 
oval area in each of the antero-lateral regions of the bladder the tunica externa 
becomes extremely thin, though still retaining its normal structure of an inner layer 
of vertically arranged fibres, and an outer stratum of curvilinear fibres. The thinness 
of these areas is principally due to the extreme tenuity of the outer stratum. The 
dorsal edges of the fibres forming each oval area are inserted into the outer rim of the 
thickened inner margin of the crescentic process of the tripus, immediately behind the 
insertion of the mesial fibres of the anterior wall into the ventral ridge of the same 
ossicle. To these oval areas the bony plates of the “ elastic-spring ” mechanism are 
so closely, and even forcibly, applied, that in all our specimens of this .species the 
antero-lateral walls of the anterior chamber were distinctly pinched in (fig. 60).'”" 
Apart from a delicate connective tissue which extends between them, the oval areas 
are not specially attached to the bony plates ; at all events, in spirit-preserved speci- 
mens the two structures readily separate from each other. The radial fibres of the 
tripus {r.f.) are somewhat feebly developed, and, from the peculiar shape of the 
crescentic process of that ossicle, pursue an almost straight course forwards as they 
pass from the latter to their connection anteriorly with the radial nodule. 
A thin transversely-disposed membrane is closely applied to the proper anterior wall 
of the bladder, and is attached dorsally to the lateral ridges of the complex centrum ; 
laterally to this the membrane becomes continuous with the margins of each oval 
plate of the “ elastic spring ” mechanism, and from thence becomes lost on the lateral 
and ventral surfaces of the bladder. 
In shape, the tripus (fig. 59, 62 and 63, tr.) is very unlike that of most other Siluridse, 
although it possesses the three characteristic divisions. The ossicle is almost straight, 
the posterior or crescentic process {tr.c.) being almost a direct continuation backwards 
of the anterior process (tr.a.), with not more than the faintest suspicion of the usual 
inward curvature. Both processes are much flattened, but the crescentic process is 
almost twice the length and width of the anterior division (fig. 62). The ventral 
surface of the root of the crescentic process is traversed by a faint oblique ridge (v.r .) ; 
the outer margin is extremely thin, and fits into a slit-like groove at the junction of the 
oval plate with the flexible root of the “ elastic spring” apparatus (fig. 59, t7\c.) ; the 
inner margin, on the contrary, is curled inwards on itself, so as to form a thickened rim 
on the ventral surface, and, moreover, is somewhat longer than the outer margin (fig. 
62). As previously mentioned, the ventral ridge of the tripus receives the insertion 
of a slip of fibres derived from the median portion of the anterior wall of the bladder ; 
the thickened inner rim of the crescentic process receives on its inner edge the 
insertion of the radial fibres from the radial nodule ; while the outer margin of the 
* Sorensen’s conclusions (37) with regard to Doras and Synodontis, viz., that the oval plates of the 
“elastic-spring” apparatus owe their formation to the ossification of “laplevre” (= the transverse 
membrane) and the tunica externa of the bladder itself, may be extended also to Auchenipterus. The 
relations of the transverse membrane to the terminal plates in this genus, and the extreme tenuity of 
the walls of the bladder where the plates are applied to them, are quite in harmony with this suggestion. 
