ON THE ANATOMY OF FISHES. 
163 
apparatus in this South American genus, the relations of the mechanism to the air- 
bladder have been carefully described by Sorensen (37) in E. nuchalis. 
Instead of each terminal disc or plate being flat on its posterior face, or only 
slightly concave, it is hollowed out in the form of a pouch. The formation of these 
singular structures and their relations to the tunica externa of the air-bladder are 
thus described by Sorensen : — “ la membrane externe de la vessie natatoire est ossifiee 
en dehors (ou en avant), et en haut, au bord du disque, I’ossification s’etend dans toute 
son epaisseur, tandis que, pres du bord superieur dudisque, cette membrane n’est 
ossifiee sur la face interne (ou posterieure) que dans une petite etendue.” 
{Loc. cit, pp. 139-140.) 
Cetopsis candira. 
Cetopsis is one of the nine genera of Siluridse in which Johannes Muller ( 28) 
denied the existence of an air-bladder, and his statement on this point remained 
uncorrected until it was shown by one of us (4) that in C. candira the organ is 
undoubtedly present in a diminutive and bone-encapsuled condition. The description 
then given may now be supplemented in one or two minor features. 
Cetopsis differs from all other Siluridje with which we are acquainted in that the 
neural arch of the complex vertebra is not perforated by a single foramen for the 
passage of the roots of spinal nerves. The second to the fourth, inclusive, emerge 
between the exoccipital and the arch of the complex vertebra, the last-mentioned 
nerve travei’sing a slight notch in the anterior margin of the arch. The fifth nerve 
escapes between the arches of the complex and fifth vertebrae, while the sixth and 
remaining spinal nerves perforate in the normal fashion the arches of the fifth and 
succeeding vertebrae. 
The transverse processes of the fourth vertebra are alone concerned in the formation 
of the osseous capsules for the enclosure of the rudimentary air-bladder, and are 
represented on each side by a hollow bony cylinder disposed at right angles to the long- 
axis of the vertebral column. The formation of each cylinder is apparently due to the 
expansion of the transverse process combined with the downwai’d curvature of its 
anterior and posterior margins, and their subsequent fusion on the ventral surface. 
The distal aperture of the cylinder is closed by the external skin and a deposit of 
subcutaneous fatty tissue, and to a slight extent also by the lateral musculature of 
the trunk. Proximally, the cylinder is continuous with the centrum and arch of the 
complex vertebra. Near its junction with the vertebra each cylinder is slightly 
bullate on its dorsal surface, but for the rest of its extent is of fairly uniform calibre. 
The bony walls of the cylinders are everywhere complete except at their distal 
extremities. Two foramina perforate the I’oot of each cylinder, one on the ventral 
surface and the other in the anterior wall, both apertures being close to the junction 
of the cylinder with the complex vertebra ; the first serves for the transmission of 
the ventral divisions of the second, third and fourth spinal nerves, while the second 
• Y 2 
