234 
PROFESSORS T. W. BRIDGE AXD A. C. HADDON 
present, it lies between the paired foramina for the exit of the dorsal and ventral roots 
of the second and third spinal nerves. 
The only variations noticed in the condition of the scaphinm relate to the occasional 
absence of an ascending process ; a globose condylar process is always present. 
Claiistra are invariably present, but vary greatly in size, from the usual condition 
of extremely slender spicules to somewhat triangular plates [Pangasius B^ichanani). 
The air-bladder has the same fundamental structure in all the S. normales. In all 
cases the organ is more or less cordate in shape, and is subdivided internally by a 
T-sliaped arrangement of a primary transverse septum and a longitudinal septum into 
three intercommunicating compartments, of which one is anterior and transversely 
disposed, occupying the anterior third of the bladder, and two posterior and lateral 
longitudinally arranged chambers, constituting the posterior two-thirds of the 
bladder. The dorsal wall of the anterior chamber is closely moulded to the ventral 
and lateral surfaces of the complex and fifth vertebral centra, including the sub- 
vertebral keel which these centra form, and also to the ventral surfaces of the 
modified transverse processes of the fourth and fifth vertebrm. The lateral portions 
of the anterior Weall of the chamber are also partially buttressed by the decurved 
anterior margins of the transverse processes of the fourth vertebra, with or without 
the aid of the expanded inferior processes of the post-temporals (post-temporal 
plates), while the median portion of the wall is not infrequently supported by a sub- 
vertebral process {e.g., Anns). The lateral compartments, on the other hand, are 
neither invested by bone, nor are they in any way directly attached to the skeleton, 
but lie free in the abdominal cavity. Except in relation to the size of the Fish, the 
variations in the capacity of the anterior chamber as compared with those of the 
lateral compartments are l)ut slight, and, as a rule, any increase or diminution in the 
relative size of the bladder is mainly due to variations in the size of the lateral 
chambers. With the exception of two genera {Rita and Aspredo) included in this 
group, the capacity of the anterior chamber is always much smaller than the combined 
capacities of the two lateral cliambei’S, and, in one of the two exceptions referred to 
•{Rita), the partial sup})ression of the lateral compartments is compensated by the 
development of two large lateral cmca from the anterior chamber. In the two genera 
last mentioned the lateral chambers are very small in one {Rita), and almost non- 
existent in the other {Aspredo), but in both cases a primary transverse septum is 
])resent, and retains its normal attachments to the skeleton. Apart from its longitudinal 
constriction into two laterally bulging halves— a separation which in some cases {e.g.. 
Arias, Callichrous, &c.) may be emphasised by the formation of one or two longi- 
tudinally arranged and inwardly projecting ridge-like aggregations of fibres from the 
median line of the posterior, ventral, and anterior walls — the cavity of the anterior 
chamber has smooth walls, and is not subdivided by the growth of internal septa. 
The lateral compartments may also have undivided cavities {e.g., Auehenoglanis, 
Callichrous, and Silurus), but not infrequently they are rendered more inexpansible. 
