1889.] Contributions to the Anatomy of Fishes. 



315 



attachment and support of the anterior wall of the air-bladder. In 

 some Siluroids the connexion of the skull with the anterior vertebras 

 may be rendered still more intimate by the articulation of the supra- 

 occipital spine with the expanded dermal plates of the first and 

 second interspinous bones, as in Auchenipterus, Oxydoras, &c, or even 

 by the downward growth of paired processes from the supraoccipital 

 to unite with the dorsal surfaces of the transverse processes of the 

 fourth vertebra, as in Arius, Batrachocephalus, &c. 



The centrum of the first vertebra varies greatly in size, but is 

 always the smallest of the anterior vertebrae. Two pit-like sockets 

 are always found on its dorsal surface for the reception of the 

 globular condylar processes of the scaphia. The complex and fifth 

 centra are the largest, or, at all events, the longest of the anterior 

 vertebrae, and, as a rule, their anterior and posterior concavities are 

 unsymmetrically developed. In nearly all cases these centra are not 

 only elongated but laterally compressed, so as to form a prominent 

 subvertebral keel, which gives rise to a deep groove along the medio- 

 dorsal line of the anterior chamber of the air-bladder, and, at the 

 same time, internally, to a prominent longitudinal ridge, partially 

 subdividing the cavity of the chamber into two laterally bulging 

 halves. A fan-shaped subvertebral process may be developed from 

 the ventral and anterior margin of the complex centrum for the 

 support of the anterior wall of the bladder (e.g., Auchenaspis) , and 

 the lateral surface of the same centrum is not infrequently thickened 

 into oblique lateral ridges for the dorsal attachment of the anterior 

 pillars of the anterior chamber. For the same purpose a variously 

 shaped osseous nodule (" radial nodule ") is attached to the dorsal 

 extremity of each ridge, or in its absence directly to the centrum, and 

 is either confluent therewith, or suturally, or by fibrous tissue only, 

 connected thereto. The radial nodules in addition to serving for the 

 attachment of the "anterior pillars" receive also the insertion of the 

 radial fibres of the tripus. Almost invariably a thin slender lamina 

 of bone, the " radial ridge," is prolonged from each radial nodule, 

 and, after passing obliquely upwards, outwards, and backwards, 

 ventral to the posterior cardinal vein, blends with the ventral surface 

 of the transverse process of the fourth vertebra. 



The neural arch of the complex vertebra is partially or completely 

 anchylused to the arch of the fifth vertebra, which, in turn, may be 

 similarly connected with the arch of the sixth vertebra, or the rigid 

 union of the different neural arches may be effected by a firm sutural 

 union. 



The transverse processes of the fourth vertebra, very frequently 

 those of the fifth vertebra, and more rarely those belonging to the 

 sixth vertebra (Platy stoma), are more or less expanded, and by their 

 partial anchylosis or sutural union with one another, form on each 



