ON THE ANATOMY OF FISHES, 
131 
of its extent the anterior margin of the process is suturally united to the posterior 
edge of the preceding process. The transverse process of the sixth vertebra is 
long and slender, free distally, but slightly overlapped proximally by the posterior 
edge of its predecessor. Its distal extremity carries the first rib (r.^). 
The expanded and otherwise modified transverse processes of the fourth, fifth, and 
sixth vertebrae, together with the superficial ossifications, form on each side a smooth, 
bony surface, concave from before backwards, and closely moulded to the convexity 
of the anterior and dorsal walls of the anterior compartment of the air-bladder. 
The strong development of the superficial ossifications and their median and ventral 
coalescence to form an aortic canal renders the subvertebral keel unusually prominent, 
and its groove-like impression in the medio-dorsal line of the anterior chamber 
exceptionally deep. 
Except for the large size of the accessory articular processes of the basioccipital 
and their union with those of the complex centrum to form the subvertebral process, 
the structure of the hinder part of the skull substantially agrees with that of 
Macrones. The cranial recesses for the utriculi are unusually spacious, and are visible 
externally as prominent rounded bullae on the surfaces of the auditory capsules near 
the postero-lateral angles of the skull. The cavum sinus imparis (fig. 46, c.s.i.) 
contracts towards its hinder extremity and communicates with the two atria by a 
comparatively small aperture. The posterior edge of its roof is produced downwards 
into a median process, which nearly meets the slightly upturned posterior margin of 
the basioccipital and strengthens the otherwise fibrous roof and posterior wall of the 
atrial cavities. The posterior face of the skull is firmly united to the anterior 
vertebrae by two broad shelving plates of bone which extend downwards from the 
lateral portions of the supraoccipital and suturally articulate with stout ridges on the 
dorsal surfaces of the transverse processes of the fourth vertebra (figs, 45, 46, so.^). 
The supraoccipital spine (w.^) is very strongly developed and extends backwards over 
the interspace between the divergent spines of the third and fourth vertebrae to its 
sutural union with the first interspinous bone or nuchal plate (i.s.). A thin lamina of 
bone is given off from the posterior plate of the exoccipital between the two foramina 
for the passage of the pneumogastric and hypoglossal nerves, and extends backwards 
towards the root of the first of the modified transverse processes ; this plate forms 
the outer wall of a recess in which the saccus paravertebralis is lodged, the inner 
wall being formed by the adjacent lateral surfaces of the first vertebral centrum and 
the anterior portion of the complex centrum (fig. 47), 
It will be observed that the union of the skull with the anterior vertebrae is 
unusually intimate in Arius. Apart from the partial anchylosis of the accessory 
articular processes of the basioccipital and complex centrum in the formation of the 
characteristic subvertebral process, the union of the paired supraoccipital laminoe and 
the median supraoccipital spine with the transverse processes of the fourth vertebra 
