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



communicating compartments, of which one is anterior and two pos- 

 terior or lateral in position. The anterior and dorsal walls of the 

 anterior chamber may be more or less completely invested by the 

 modified transverse processes of the fourth and fifth vertebrae, but 

 the latter do not form deep concave recesses or capsules for the 

 partial or complete enclosure of the entire air-bladder. 



In the Siluridas abnormales on the contrary, the air-bladder is 

 always small relatively to the size of the Fish, and more or less dege- 

 nerate, sometimes partially solid, but almost invariably includes two 

 laterally situated air-sacs with simple cavities, which together may 

 be regarded as equivalent to the anterior chamber of a normal 

 Siluroid. Lateral compartments, as a rule, are absent altogether, or, 

 if present, are very rudimentary. Whatever its condition, the air- 

 bladder is almost always partially or completely enclosed within 

 transversely disposed bony recesses, formed either by the transverse 

 processes of the fourth vertebra alone, or in conjunction with those of 

 the fifth vertebra. 



Although a convenient method of classifying morphological facts, 

 it is obvious that this classification, based as it is upon so variable an 

 organ as the air-bladder, can have no genetic value. 



(I.) Siluridee Normales. 



Under this head may be included such genera as : — Plotosns, 

 Wallago, Callichrous, Cryptopterus (certain species). Eutropius, Pan- 

 gasius, Macrones, Rita, Pimelodus (certain species), Piramutana, Arias, 

 Otteogeniosus, Oxydoras, Malapterurus, and others. 



In the Siluroids included in this group the number of rigidly inter- 

 connected vertebra? varies. The first, the complex, and the fifth 

 vertebras are almost invariably so connected together that no motion 

 is possible between them, and occasionally the sixth, the seventh, and 

 even the eighth may be included in the series. The rigidity of the 

 complex and fifth vertebras, with the occasional addition of the sixth, 

 may be further increased by the sutural union or partial anchylosis 

 of their respective transverse processes. The anterior vertebras are 

 also firmly connected to the skull, generally by the articulation of the 

 arch and spine of the third vertebra with the exoccipitals and 

 supraoccipital, the transverse processes of the fourth vertebra with 

 the post-temporals, and the spinous processes of the third and fourth 

 vertebras with the supraoccipital spine ; less frequently by the forma- 

 tion of interlocking accessory articular processes on the contiguous 

 ventral margins of the basioccipital and the centra of the first and 

 complex vertebras {Macrones, Bagrus) . In Arius the accessory pro- 

 cesses are very strongly developed, and by their downward growth 

 and coalescence, form a stout, conical subvertebral process for the 



