90 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
in what light it should be regarded. In some i-espects it resembles a backwai'd 
extension of the aponeurotic membrane. Ramsay Wright (43) in his description 
of the skeletal attachments of the anterior part of the air-bladder in Ainiurus catiis, 
evidently regards it as a portion of the proper anterior wall ; at all events he 
describes the dorsal wall of the anterior chamber as being anteriorly attached to an 
oblique ridge on each side of the complex centrum, and to the thickened anterior 
margin of the transverse process of the fourth vertebra, and the only portion of the 
air-bladder that answers to this description is the transversely-disposed membrane in 
connection with the proper anterior wall. McKenzie (23) refers to it as an aponeu- 
rotic membrane separating the “ head-kidney ” from the air-bladder. On the other 
hand, we have but little doubt that the membrane described by Sorensen (37) 
as more or less completely investing the air-bladder in certain Siluroids, and often 
becoming much thicker anteriorly than elsewhere, is identical with the one now 
under discussion. Sorensen, however, apparently regards the membrane as con- 
stituting an inner layer of the peritoneal coat, although from his account it would 
seem to furnish a much more complete investment to the bladder than the outer 
layer, which carries the ccelomic epithelium, and is restricted solely to the ventral 
surface. For our own part we incline to the opinion that the membrane in question 
owes its existence as a definite structure to a delamination of the proper anterior 
wall of the air-bladder, which has also affected, but less obviously, the adjacent 
lateral, ventral, and dorsal walls, and ought, therefore, to be regarded as an 
integral portion of that organ. It is also more closely adherent to the proper wall 
of the air-bladder than to the investing peritoneum, and in some Siluridse is much 
more obvious than in others, being traceable for a greater distance on to the lateral 
and ventral walls. In Arius and some other Siluridse, in addition to the membrane 
now under consideration, which is also j^i'esent, a second layer, anatomically and 
histologically undoubtedly a portion of the anterior wall of the bladder, may separate 
dorsally from the inner portion of the wall, and acquire skeletal attachments similar 
in almost eve)y respect to those of the former. It is perhaps a matter of no great 
importance which view be taken, but from a physiological point of view the 
membrane is of some interest, inasmuch as its extensive attachments to rigid 
})ortions of the axial skeleton render it an efficient and unyielding buttress to the 
proj)er anterior wall of the liladder. As we shall frequently have occasion to refer to 
the extension of ossified deposit from adjacent skeletal structures into this membrane, 
more especially in the Silurida} abnormales, we shall in future refer to it as the 
“ superficial coat ” of the air-bladder, distinguishing, however, that portion of it 
which specially buttresses the proper anterior wall as the “ transverse membrane.” 
After the removal of its ventral wall the cavity of the air-bladder is seen to be 
divided by a characteristic T -shaped arrangement of two principal internal septa, one 
of which is transverse and the other longitudinal, into three main compartments. 
Of these compartments one is anterior and transversely disposed, and the other two 
