ON THE ANATOMY OF FISHES. 
217 
wall of the air-bladder, and, dorsally, is firmly attached to the ventral surface of the 
anterior end of the complex centrum, to its oblique lateral ridges, and to the dorsal 
laminae ; laterally to this point, on each side, the dorsal edge of the membrane blends 
with the floor of the saccus paravertebralis, while its outer edge becomes continuous 
with the inner and ventral margins of the oval plate of the “ elastic-spring 
apparatus ; on the outer edge of each plate, with which it is also continuous, the 
membrane reappears and finally blends with the ligamentous fibres connecting the 
post-temporal and clavicle. Dorsally and anteriorly the membrane seems to be 
partially continuous with the aponeurotic membrane (fig. 91, ap.m.), which, after 
investing the dorsal surface of the anterior section of the mesonephros in the usual 
manner, and extending over the ventral surfaces of the inferior limbs of the post- 
temporals and the basioccipital, ultimately becomes continuous with the dorsal 
extension of the pericardio-abdominal septum. Along its entire ventral margin the 
transverse membrane is reflected backwards on to the ventral surface of the air- 
bladder, where it soon ceases to be traceable. 
The relations of the oval plates of the “ elastic-spring ” mechanism to the transverse 
membrane, combined with the almost complete disappearance of the tunica externa 
where the plates are applied to the wall of the anterior chamber, suggests 
that Sorensen’s conclusion as to the mode of formation of the plates in Synodontis 
and Doras is equally applicable to Pangasius, and that these structures in the last 
mentioned Siluroid result from the ossification of both the tunica externa and the 
transverse membrane. 
There are certain discrepancies between the brief descriptions of the air-bladder of 
this species given by Taylor and Day, and that given by us, to which reference 
must be made. Taylor’s account (38) of the bladder of Pimelodus pangasius,” 
Ham. Buch. (= Pangasius Buchanani, Cuv. and Val.), is to the effect that 
the organ is composed of four or more portions extending in a line from 
opposite the pectoral fins to near the end of the tail. The first is said to be 
generally oval, the second pyramidal, and the two last, which run between the 
caudal portions of the lateral muscles, approach to a cylindrical shape. The numerous 
septa on its internal surface are described as descending from above downwards in 
the first portion, in the second they are transversely arranged, but in the posterior 
ones form a number of irregular cells. Day’s account (Z.c., p. 700) is in the following 
terms: — “Air-vessel large, extensive, and divided into thi-ee portions. The anterior 
is somewhat heart-shaped, considerably the largest, and extends from the commence- 
ment of the vertebral column to nearly opposite the posterior extremity of the 
pectoral fin. Its remaining portions are narrow, compressed, and continued to 
opposite the middle of the anal fin, amongst the muscles covering the h?emal sj)ines 
It then becomes narrow and reduplicated on itself for a short distance. On removing 
the front wall of its first or largest portion, its interior is seen to consist of two pear- 
shaped cavities, the bases of which are inferior and lateral, whilst they coalesce 
2 F 
MDCCCXCin. B 
