152 Profs. T. W. Bridge and A. C. Haddon. [June 16, 



rudimentary but lacks even a trace of the compressor and tensor tri- 

 podis muscles which are so characteristic of the normal Pimelodinee. 

 Probably in such instances the degeneration of the air-bladder is due 

 to the assumption of a ground habit. 



The invariable persistence of the Weberian ossicles in an almost 

 structurally complete but functioiiless condition may be explained by 

 the absence of any potent cause calculated to bring about their total 

 suppression. 



The uniform retention of the anterior chamber in these Siluroids 

 while the lateral compartments have almost invariably disappeared, 

 in place of the entire suppression of the air-bladder which occurs in 

 most other Teleostei whenever that organ from similar causes has be- 

 come useless to its possessor, is due to its connection with the persistent 

 Weberian ossicles. 



The encapsulation of the air-bladder by bone is difficult to explain 

 satisfactorily, but two alternative suggestions may be made. (1.) En- 

 capsulation varies greatly in extent as well as in the precise methods 

 by which it has been brought about, but very often appears to bear 

 some relation to the structural completeness of the diminutive air- 

 bladder and the retention of its apparently functional connexion with 

 the Weberian ossicles. Where the air-bladder retains much of its 

 structural integrity encapsulation is always more complete than when 

 the contrary is the case, and may then be due to the necessity of pre- 

 venting any distension of the reduced and useless organ by varying 

 external pressures from imparting disturbing and useless stimuli to 

 the internal ear. On the other hand, encapsulation is always less 

 complete when structural lesions are too obvious to admit of any 

 possibility of pressure variations affecting the Weberian ossicles, and 

 the fact that it exists at all in such cases may be explained by the 

 supposition that reduction in the size of the only portion of the air- 

 bladder that persists, that is, the anterior chamber, has been accom- 

 panied by a corresponding contraction and curvature of the modified 

 transverse processes which normally invest, and are closely moulded 

 to, its anterior and dorsal surfaces. (2.) Encapsulation may be due 

 in part to the tendency of the transverse processes to contract round 

 and envelope an atrophying air-bladder, and in part also to an un- 

 checked development of that tendency to ossification of the walls of 

 the air-bladder, or of the investing connective tissue, which to a re- 

 stricted extent is so characteristic a feature even in the normal 

 Siluroids. 



Of the remaining families of the Ostariophysese, the Cyprinidse 

 exhibit a substantially similar and parallel series of modifications in 

 the condition of the air-bladder, which, it can scarcely be doubted, 

 are also correlated with the assumption of a grovelling and purely 

 " ground " habit of life, and, as in certain Siluridse abnormales, the 



