ON THE ANATOMY OF PISHES. 
145 
for the dorsal aorta, the superficial ossifications invest and considerably thicken the 
lateral surfaces of the complex and fifth vertebral centra, and, probably as the result 
of a further extension of ossification into the dorsal portion of the aponeurotic 
membrane, extend even on to the lateral and ventral surfaces of the basioccipital, 
completely obscuring all external indication of the body of the first vertebra, and at 
the same time leading to the complete anchylosis of the base of the skull with the 
anterior vertebral centra. Faint traces of the intervertebral sutures between the 
centra of the complex and fifth vertebrae, and between the former and the basioccipital, 
may, however, be seen on the ventral surface (fig. 53). On each of the lateral surfaces 
of the complex centrum the superficial ossification is thickened into two longitudinal 
and parallel ridges, one lying dorsad to the other with a groove between them. The 
ventral ridge coincides with the ventro-lateral margin of the centrum, and is shorter 
than the dorsal ridge. Opposite the root of the spout-like transverse process the 
ventral ridge is produced outwards into a slender, triangularly-shaped, ventral process, 
the pointed apex of which is inserted into a thin, but tough, fibrous membrane 
investing the ventral wall of the lateral air-sac of its side (v.p.). The dorsal ridge 
{d.7\) appears to commence as a prolongation of the anterior margin of the transverse 
process of the fifth vertebra, and from thence is continued somewhat obliquely inwards 
and forwards along the side of the complex centrum, immediately beneath the root of 
the modified transverse process and dorsad to the ventral ridge, as far as the basi- 
occipital, where it finally dies away beneath the lower lip of the external atrial 
aperture. The groove between the two ridges, w^hich is much better marked on the 
right than on the left side, probably transmits the posterior cardinal vein and the 
paired anterior lobes of the mesonephros. A third ridge is formed by a thin prolon- 
gation of the anterior margin of the root of the modified transverse process, along the 
lower border of the forwardly-inclined neural arch of the third vertebra, as far as the 
upper lip of the external atrial aperture. Where this ridge and the dorsal ridge 
overlap, the former is dorsad to the latter, and in a narrow slit-like groove between 
the two the anterior process of the tripus (tr.a.) passes forwards to its attachment to 
the scaphium anteriorly. The external atrial aperture is situated at the anterior 
extremity of this groove, and near its hinder end the groove deepens into a pit, at 
the bottom of which the articular process of the tripus articulates with the lateral 
, surface of the anterior third of the complex ceotrum ; posteriorly, the groove opens 
into the recess enclosed by the spout-like transverse process. 
We were unable to identify with certainty the usual oblique ridges on the sides of 
the complex centrum, or dorsal laminae, neither could we detect any radial nodules. 
It may not improbably be the case, however, that the well-marked ridge (dorsal ridge) 
which leaves the ventral surface of each of the transverse processes of the fifth 
vertebra, and extends obliquely forwards and inwards along the adjacent lateral 
surface of the complex centrum, may represent both the lateral ridge and dorsal 
lamina of other and more normal Siluridae ; but if this be so, it is somewhat 
MDCCCXCIIl. — li. v 
