ON THE ANATOMY OP FISHES. 
141 
ventral surface may be compressed by the contraction of two oblique muscles. If 
Cuv^iER and Valenciennes are correct in their description, the two oblique muscles 
may possibly correspond to the large compressor muscles which we have already 
described in the genera Platy stoma, Piinelodus, and Piramutana. We have had no 
opportunity of examining this particular species, but we may safely assert that no 
such muscles, either dorsal or ventral, are present in PE. longispinis. 
Batrachocephalus mino. 
In no essential feature is there any difference between the air-bladder and skeleton 
of this species and those of Anus pidada. 
Relatively to the size of the body the air-bladder is rather large, and this is 
noticeably the case with the anterior chamber, which is both long and deep. Its 
walls are comparatively thin, and anteriorly are in close and extensive contact with 
large lateral cutaneous areas. Three pairs of secondary transverse septa incompletely 
subdivide the cavities of the two lateral compartments. 
The general structure of the organ is represented in fig. 52, including the dispo- 
sition and skeletal attachments of the principal sheets of fibres in the walls of the 
anterior chamber. In the left half of the compartment the arrangement of the fibres 
of the inner stratum of the tunica externa [in.st.) may be seen ; on the right side 
the inner stratum has been removed so as to show the characteristic attachment of 
the outer stratum of the anterior wall {o.st.) to the transverse process of the fourth 
vertebra and the curvilinear disposition of the fibres of the same stratum in 
the lateral wall, and also their convergence in the dorsal wall in the form of a 
triangular sheet {o.st.') to their insertion into the crescentic process of the tripus. 
Osteogeniosus Valenciennesii. 
In most of its osteological details this species'"' is almost identical with Arius 
pidada. 
The air-bladder (fig. 51) is conical in outline, with a broad anterior, and a somewhat 
oval posterior, extremity. The anterior wall is rendered deeply emarginate in the 
median line by the impression of the sub vertebral process {sv.g.) to which it is 
somewhat firmly attached by fibrous tissue. Internally the bladder is subdivided 
into anterior (a.c.) and lateral compartments (Z.c.) by the usual transverse (t.s.) and 
longitudinal (^.s.) septa. The transverse septum has the normal attachment to the 
skeleton along its contracted dorsal edge, but towards the ventral margin widens 
considerably, and laterally is directly continuous with the ventral wall of the bladder ; 
* The only reference to the air-bladder of this Indian Siluroid that we are acquainted with, is by 
Day (9). He says, “ In a specimen from Moulmeiu, taken in the river, the air-vessel was large, heart- 
shaped, having an internal longitudinal septum, and not enclosed in bone.” 
