1889.] 



Contributions to the Anatomy of Fishes. 



321 



As tlie anterior wall is usually more or less efficiently buttressed by 

 the transverse processes of the fourth vertebra, or by post-temporal 

 plates, or median subvertebral processes, the extent of its attachment 

 to the skeleton varies inversely with the extent to which it is invested 

 or supported by bone. The median portion of the wall is always 

 attached dorsally to the ventral surface and sides of the anterior 

 portion of the complex centrum, often by means of laterally situated, 

 oblique, bony ridges, and also to the radial nodules. Laterally to this, 

 on each side, the anterior wall may be so completely invested by bone 

 as to be free from any special connexion or attachment to rigid 

 portions of the axial skeleton {e.g., Maerones) ; or in correlation with a 

 less complete bony support, the outer stratum of the tunica externa 

 of the anterior wall may separate dorsally from the inner stratum and 

 become firmly inserted into the decurved anterior margin of the 

 transverse process of the fourth vertebra (e.g., Arius, Auchenasyis, 

 Pimelodus). The dorsal attachment of the median portion of the 

 anterior wall to the radial nodules and the complex centrum occurs in 

 all the normal Siluroids, and may be regarded as constituting the 

 " anterior pillars " of the compartment. The ventral wall may also be 

 considered as rigidly attached to the skeleton, both in front and behind, 

 inasmuch as its inner stratum of longitudinally disposed fibres, some- 

 times thickened into stout inwardly projecting ridges, extends into 

 both the anterior and posterior walls, and shares the skeletal attach- 

 ments of the anterior and posterior pillars. Although, as a rule, 

 extremely thin, the median portion of the dorsal wall, over an area 

 bounded in front and behind by the anterior and posterior pillars, and 

 laterally by the dorsal walls of the two bulging halves of the chamber, 

 is always firmly attached to the ventral and lateral surfaces of the 

 complex centrum, and possibly also to those of the fifth centrum. 



The attachment of the walls of the anterior chamber to moveable 

 ossicles (the tripodes) is effected by the convergence of the fibres of 

 the anterior and lateral walls into the dorsal wall in the form of two 

 triangular sheets, and their ultimate insertion into the crescentic 

 processes of the tripodes, which are situated near the anterior and 

 inner corners of the lateral halves of the anterior chamber. The 

 variations in the extent to which these fibres are attached to the 

 tripodes are mainly confined to one feature. A slip of fibres derived 

 from the median portion of the anterior wall is always inserted 

 dorsally into the ventral ridge of each tripus, or directly into the 

 ventral surface of the ossicle when the ridge fails to be developed. 

 Laterally to this point the fibres forming the whole thickness of the 

 tunica externa of the anterior and lateral walls may converge in the 

 dorsal wall and become attached to the tripodes (e.g., Maerones) ; or 

 as in many other Siluroids (e.g., Arius, Pimelodus, &c.) the outer 

 stratum of the anterior wall is continuously attached by its dorsal 



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