120 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
they become iu dined obliquely forwards towards the ventral surface of the bladder, 
while their dorsal margins curve forwards parallel to each other along the sides, and 
eventually along the ventral surface of the fifth vertebral centrum, to ’which they 
become firmly attached. Near the anterior end of the centrum the lateral halves of 
the septum unite at an acute angle in the median line, and form a forwardly inclined 
keel projecting into the cavity of the anterior chamber, and partially subdividing it 
into two lateral divisions. The anterior portion of the longitudinal septum makes a 
slight deviation from the median line, and unites with the posterior face of the left 
half of the primary transverse septum ; hence it is that the right lateral compartment 
extends forwards between the opposed faces of the converging lateral halves of the 
transverse septum until the latter finally unite in the median line anteriorly, 'fhese 
features are more or less characteristic of other species of Fimelodus with normal 
air-bladders [vide P. ornatus, fig. 36) and of Piramutana, but they are particularly 
well marked in P. macidatus. 
The divergence of the dorso-lateral and ventro-lateral muscles of the trunk is so 
slight that scarcely any portion of the lateral walls of the anterior chamber is in 
contact with the external skin, and the two structures are further separated by a 
considerable amount of fatty tissue. 
In an immature specimen of this species about 4 inches in length, the air-bladder 
had extremely thin walls, and its anterior chamber was in close and extensive 
relations with lateral cutaneous areas, which externally presented the appearance of 
superficial blisters of the skin. The longitudinal septum was attached in front to the 
right half of the primary transverse septum instead of the left, and hence the riglit 
lateral compartment was prolonged forwards on the dorsal or posterior face of the 
inclined septum, between its converging lateral halves. 
A compressor and a tensor tripodis muscle were present in i-elation Avith each half 
of the anterior chamber. 
Pimelodus ornatus. 
In this species also the air-bladder and anterior vertebrae are essentially similar to 
those of Piramutana. 
The anterior wall of the bladder (fig. 36) is somewhat eniarginate in the median 
line, and consequently the anterior chamber presents the appearance of two sub- 
globose sacs continuous with each other aci’oss the median line, and with the two 
lateral compartments posteriorly. Relatively to its length and width, the bladder, 
and especially its anterior chamber, are somewhat deeper than in Piramutana. Its 
Avails are extremely thin. Two pairs of secondary transverse septa {t.s.') are present, 
as also are compressor and tensor tripodis muscles. 
