220 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
The air-bladder itself has thin walls, and consists of an anterior and somewhat 
rounded portion, which includes the anterior and two lateral compartments, and in 
addition also, a posterior much flattened, leaf-like caecum connected with the 
foregoing by a short tubular stalk. The cavities of both the lateral compartments 
and the posterior csecal appendage are largely occupied by a trabecular network of 
branching columns of fibres, but the outer portions of the former are comparatively 
free from such bundles, and communicate in front with the anterior chamber. As in 
P. djamhal this network seems to owe its formation to the partial resolution of a 
series of secondary transverse septa into branching bundles of fibres. The wall of 
the anterior chamber is not specially thin where the oval plates of the elastic- 
spring ” mechanism are applied to it, and it may therefore be inferred that, as in 
Malapterurus, according to Sorensen, the plates result from the ossification of the 
transverse membrane alone, the tunica externa having no share whatever in their 
formation, 
Pangasius micronema. 
In a specimen from Dr. Bleeker’s collection labelled P. micronema, and agreeing 
in every particular with the description of this species in the British Museum 
Catalogue of Fishes (vol. 5, p, 63), we noticed one or two features in which it 
presented a striking contrast to all other species of the same genus that had come 
under our notice. 
Each of the transverse processes of the fourth vertebra (fig. 96) has a broad 
flat root, which is prolonged distally into distinct anterior {t.p^a.) and posterior 
{tp.^p.) divisions, separated from each other by a broad but comparatively shallow 
cleft. The anterior division {t.pda.) is thick and inflexible, tapering somewhat 
towards its decurved distal extremity, but is without a trace of the oval plate so 
characteristic of other species of Pangasius ; on the contrary, and as in most other 
Siluridse not provided with an “ elastic-spring ” mechanism, the distal portion of 
the process is applied to, and firmly supports the outer extremity of the inferior 
limb of the post-temporal (pt.i.), and also the inner of the two processes into which 
the cleft stem of that bone {pt.s.) divides in forming the socket for the clavicle 
(c.^). Hence, in this species, the transverse process of the fourth vertebra is not 
modified to form an “ elastic-spring ” apparatus, but both in shape and in the support 
which it aflbrds to the post-temporal and pectoral girdle, closely conforms to its 
normal condition in the great majority of the Siluridae ; and in harmony with this 
modification, is the fact that the inferior limb of the post-temporal is relatively 
slender, or at all events is nothing like so massive as in other species of 
Pangasius, 
The air-bladder is broadly ovate in shape but rendered slightly bilobed in front 
through the existence of a deep median notch in its anterior wall. Relatively to the 
lateral compartments the anterior chamber is very small, and there is no posterior 
