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PROFESSORS T. W. BRIDGE ANT) A. 0. HADDON 
latter are usually connected with the post-temporals. The dorsal margin of the 
membrane is always more or less continuous with the fibrous floor of the saccus 
paravertebralis. Where an “elastic-spring” apparatus is present the membrane is 
continuous with the margins of the terminal plates, and from thence is continuous 
with the connective tissue investment of the lateral and ventral surfaces of the 
anterior chamber of the air-bladder. In those Siluroids in which subvertebral 
processes are present the transverse membrane is always attached to their lateral 
and ventral margins.^' Anteriorly, the dorsal margin of the transverse membrane 
is more or less obviously continuous with an aponeurotic membrane which, after 
investing the dorsal surface of the “ head-kidney,” and the ventral surfaces of the 
centrum of the first vertebra and the hinder portion of the basioccipital, extends 
ventrally as a transversely disposed fibrous sheet in relation with the anterior 
surface of the “head kidney,” and laterally blends with the ligamentous fibres that 
unite the post-temporals with the transverse processes of the fourth vertebra. 
This aponeurotic membrane we regard as a dorsal and backward extension of 
the same fibrous sheet that, ventrally, separates the pericardial and abdominal 
cavities. On the contrary, the connective tissue investment of the air-bladder and 
its special development, the transverse membrane, must be considered as belonging 
to that organ, of which it is really the superficial coat. These structures are of 
some interest for two reasons — first, because they ai'e liable to become the seat 
of ossified deposit ; and, secondly, because the inextensibility and the skeletal 
attachments of the transverse membrane enable it to supplement other structures 
in affording a rigid support to the anterior wall of the bladder. 
In almost all normal Siluroids the lateral or outer walls of the anterior chamber of 
the air-bladder are more or less extensively and intimately applied to lateral cutaneous 
areas, and this relation of the two structures is always brought about by the diver- 
gence of the dorso-lateral and ventro-lateral muscles of the trunk combined with an 
unusual lateral extension of the anterior portion of the bladder. 
In all the normal Siluroids, without an exception, a ductus pneumaticus is present, 
and, after a sigmoid or slightly tortuous course, opens into the anterior chamber in 
the median line of its ventral wall, and immediately in front of the ventral margin of 
the primary transverse septum. 
The general structure of the walls of the air-bladder in all normal Siluroids is 
essentially similar to that we have already described in Macrones (p. 92), but their 
thickness is subject to considerable variation in different species. The fundamental 
arrangement of the fibres of the tunica externa in the form of an inner stratum of 
circularly disposed fibres and an outer stratum of longitudinal fibres is always so far 
* In a few genera, and notably in the normal Pimelodinse, no distinct transverse membrane exists, 
and in such cases it seems probable that the membrane has coalesced with the proper anterior wall of 
the bladder, and shares with the latter its dorsal attachment to the anterior margins of the transverse 
processes. 
