ON THE ANATOMY OF FISHES. 
151 
plate of the exoccipital, especially where they form the external walls of the fovese 
sacculi. 
The fovese sacculi, the cavum sinus imparis, and the atrial cavities appear to 
differ in no essential respect from the corresponding structures in other Siluridse. 
The foveas are, perhaps, somewhat larger than usual, while, on the contrary, the cavum 
sinus imparis is relatively smaller. Owing, it may be, to the large size of the foveas, 
the cranial portion of the basioccipital is unusually wide, and, as the cavum and its 
atrial diverticula are relatively smaller than usual, the atria and their external aper- 
tures with the scaphia lie together within the cranial cavity and on the dorsal surface 
of the hinder part of the basioccipital. The large size of the fovese also causes them 
to approach each other in the median line beneath the cavum sinus imparis, so that 
only a very thin vertical lamina of bone separates the two cavities. 
As in Bagarius the air-bladder is completely constricted into two laterally situated 
oval sacs (fig. 57, a.s,). Each sac occupies and completely fills the central and 
similarly shaped portion of the recess enclosed by the modified transverse process of 
the fourth vertebra, and even projects ventrally beyond it, but nevertheless remains 
separated from the complex centrum by the length of the flattened root of the process. 
Externally the sac does not extend quite to the distal aperture of its bony recess, 
which to a slight extent is closed by the stem of the post-temporal, but principally by 
the lateral cutaneous area {I.c.a.) of its side.'^' The ventral surface of each air-sac is 
closely invested by the anterior portion of the mesonephros which, at the antero- 
internal angle of the sac, is prolonged inwards and then backwards, and, accompanied 
by the posterior cardinal vein, passes between the inner wall of the sac and the dorsal 
lamina externally, and the lateral surface of the complex centrum internally. The 
mesonephros in conjunction with a portion of the lateral lobe of the liver, also fills 
up the interval between the outer extremity of each air-sac and the inner surface of 
the corresponding lateral cutaneous area. The superficial coat of the air-bladder is 
but feebly developed, although it has skeletal attachments similar to those already 
described in the case of Bagarius, and may also be traced across the ventral surface 
of the complex centrum as it extends from the ventral surface of one air-sac to that 
of the other. The inner third of the under surface of each sac is overlapped by the 
distal portion of the ventral process (fig. 57). 
The attachments of the walls of each air-sac to rigid portions of the skeleton are 
in the main similar to those of Bagarius. Thus, each sac is connected with the 
walls of its bony recess as follows : — (i.) along the dorsal edge of its posterior wall 
by an outer stratum of fibres which apparently becomes detached from the rest and 
is inserted into the posterior margin of the denn-cylindrical transverse process ; (ii.) 
along the corresponding line of the anterior wall by the insertion of the dorsal edge 
of the latter into the decurved anterior margin of the same process ; (iii.) the ventral 
* Tbe length of each sac is 4 mm. and the "width mm., the length of the Fish itself being five 
inches. 
