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Profs. T. W. Bridge and A. C. Haddon. [June 20, 



shaee an elongated oval body (Pangasius djambal, Malapterurus 

 electricus), or it may be flattened and leaf -like (Pangasius macronema). 

 In one instance (Oxydoras) it is very rudimentary. The existence of 

 a pair of rudimentary posterior caeca is very rare (AucJienipterns 

 ohscurus). Very generally the longitudinal septum extends backwards 

 into the posterior caecum, and subdivides its cavity into two distinct 

 lateral canals or chambers, which communicate anteriorly with the 

 proper lateral compartments of the bladder. Not infrequently the 

 single or double cavity of the caecum is partially subdivided internally 

 by a series of circularly disposed, inwardly projecting ridges (e.g., 

 Malapterurus) . In some Siluroids (Pangasius), where the lateral 

 chambers are largely occupied by a trabecular network of fibrous 

 bundles, the cavity of the posterior caecum is largely obliterated by 

 the formation of a similar growth. It may be remarked that in 

 nearly all the Siluroids with an " elastic-spring-apparatus " that we 

 have examined, posterior caeca were present, although in regard to 

 their size and the extent to which their cavities are subdivided, or 

 partially obliterated, cousiderable variety exists. In two species only 

 (Auchenipterus nodosus and Pangasius micronema) are these structures 

 entirely absent. 



In all the normal Siluroids, without an exception, a ductus pneu- 

 maticus is present and 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. 



Not only is the anterior compartment of the air-bladder more or 

 less completely invested by bone on its dorsal and anterior surfaces, 

 but its walls are attached to rigid portions of the axial skeleton and 

 to movable ossicles at certain special points. As to the nature and 

 extent of the fixed skeletal attachments, there is substantial uni- 

 formity in the different members of the group, and the physiological 

 effect of such skeletal connexions is, in the great majority of cases, 

 the same, viz,, to render the anterior, dorsal, ventral, and posterior 

 walls incapable of participating in any distension of the chamber, 

 which, consequently, must solely depend upon the movements of the 

 lateral walls. The posterior wall, i.e., the primary transverse septum, 

 is always attached by its dorsal margin to the ventral and lateral 

 surfaces of either the complex or the fifth centrum — rarely to the sixth 

 centrum ; laterally to this the dorsal edge of the septum is invariably 

 attached to the ventral surfaces of the transverse processes of the fifth 

 vertebra, or to those of the fourth vertebra, or exceptionally to the 

 corresponding processes of the sixth vertebra ; and, in addition, a 

 sheet of fibres is generally prolonged forwards, on either side of the 

 complex centrum, into the dorsal wall, where it eventually becomes 

 attached to the radial ridge of its side. We propose to speak of these 

 attachments as forming the i{ posterior pillars " of the compartment. 



