ON THE ANATOM T OF FISHES. 
117 
the anterior chamber the fibres of the same stratum may be traced into the dorsal 
wall, where they form a thin layer on the inner surface of the curvilinear fibres of the 
outer stratum but, instead of extending inwards to their usual insertion into the 
tripodes, thin away near the inner edges of the triangular sheets. The insertion of 
the tendon of each of the small extrinsic muscles of the bladder into the anterior wall 
of the anterior chamber exactly coincides with the position of those fibres of the inner 
stratum of the tunica externa, which are dorsally inserted into the inner margin 
of the root of the crescentic process of the tripus, and is in close contiguity with the 
similar insertion of the radial fibres of that ossicle. As the contraction of these 
muscles would probably have the effect of tightening such fibres, and, therefore, 
restrict the lateral movements of the tripodes, we venture to suggest for each the name 
of “ tensor tripodis.” The radial fibres of the tripus are strongly developed, but 
otherwise are normal. 
Apart from the outer stratum of the anterior wall of the bladder, which is dorsally 
attached to the complex centrum, to the radial nodules and to the dorsal laminae, as 
well as to the transverse processes of the fourth vertebra, we could detect no equivalent 
to the transverse membrane of other Siluroids. It is possible, however, that the outer 
stratum includes, not only a portion of the tunica externa of the anterior wall, but 
the transverse membrane as wmll ; at all events, such an explanation is suggested 
by the fact that the outer stratum has the characteristic attachment of the transverse 
membrane to the dorsal laminrn, which is, apparently, never the case with any 
portion of the proper tunica externa in other Siluroids. All the normal Pirnelodinae 
that we have examined resemble Platystoma in this respect. 
A small csecal appendage is closely attached to each of the lateral walls of the 
anterior chamber, and is partially overlapped by the great compressor muscle of its 
side (figs. 33 and 34 , al.c.). The cavity of this curious appendage communicates with 
that of the anterior chamber by means of a number of slit-like apertures in the outer 
wall of the latter (fig. 34 ). Anteriorly, each caecum separates from the wall of the 
bladder, and expands into a pear-shaped enlargement, which is in contact with the 
inner surface of the dorsal extremity of the clavicle. The cavity of the appendage is 
broken up into a series of intercommunicating spaces by a network of fibrous bundles, 
and the tunica interna is prolonged outwards from the anterior chamber through the 
slit-like orifices in its outer wall, so as to line the interior of the coecum and the 
spaces enclosed by the fibrous network. 
The claustra are well developed and lie one on each side of the foramen magnum ; 
each ossicle is strongly curved in conformity with the shape of the aperture which 
it partially encircles. The scaphiura has the usual shape, but its spatulate process 
is relatively small and greatly thickened ; the ascending process is short and very 
slender. The intercalarium is represented by a thick nodule in the inter-ossicular 
liffament. The tripus also is normal in shape ; its anterior process, however, is 
slightly longer than the crescentic process. The latter portion of the ossicle has a 
