AIR-BLADDER IN NOTOPTERUS BOESEEXSIS. 511 



The dorso-laleral margins of the abdominal portion o£ the air- 

 bladder (PI. 36. fig. 3) are firmly attached to the transver&e 

 processes of certain of tlie trunk vertebrae and to the proximal 

 portions of their costal elements, the latter being so intimately 

 related to the bladder as to produce a series of faint trans- 

 versely-disposed grooves in the lateral walls of that organ. As 

 previously stated, the peritoneum invests the ventral surface of 

 this portion of the air-bladder ; and it may be added, that in this 

 region it assumes the condition of an exceptionally tough fibrous 

 membrane which is firmly adherent to the veiitral wall of the 

 bladder. Posteriorly, the peritoneum is reflected downwards on 

 to the haemal arch and spine of the first caudal vertebra and the 

 first radial element of the anal fin, and is, moreover, firmly 

 attached to these skeletal structures. Hence it follows, that the 

 ventral wall of the abdominal section of the air-bladder is firmly 

 attached posteriorly to the anterior caudal skeletal elements. 

 The walls of this portion of the bladder are of moderate and 

 equal thickness throughout. Internally, the cavity of the 

 bladder is subdivided into two lateral compartments by a vertical 

 longitudinal septum, which is continuous dorsally and ventrally 

 with the corresponding walls of the bladder (fig. 3, l.s.). Poste- 

 riorly, the septum increases in height with the increasing 

 vertical dimension of the bladder, and, at the point where the 

 latter subdivides into the two caudal cseca, the hinder margin of 

 the septum is inserted into the anterior face of the hgemal arch 

 and spine of the first caudal vertebra and the proximal portion of 

 the first radial element of the anal fin. Anteriorly, the longi- 

 tudinal septum deviates from the median plane towards the left 

 side, and, so far as its ventral portion is concerned, the septum 

 ceases immediately behind and a little to the right side of the 

 internal aperture of the ductus pneumaticus (l.s., d.p.). At this 

 point the dorsal portion of the septum, which, it may be men- 

 tioned, extends forwards into the subspherical sac, is connected 

 by a narrow obliquely-transverse septum (t.s.) with the left 

 lateral wall of the bladder along the Ime of the external oblique 

 groove {o.ff.) separating the sac from the abdominal portion of 

 the bladder*. The ettect of this singular unsymmetrical dis- 

 position of the longitudinal septum, and the presence of an oblique 

 transverse septum on tlie left side only, combined with the 



* In fig. 3 that portion of the left lateral wall of the bladder which is 

 traversed by the oblique groove is indicate. I as a slender strip. 



