144 
PROFESSORS T. W. BRIDGE AjS^D A. C. HADDON 
of the complex vertebra (fig. 53, c.c.) is very long, and its deep and almost equal 
anterior and posterior concavities nearly meet in the centre of the bone. The fifth 
vertebral centrum (v.^) is much smaller, and although somewhat larger than the 
normal centra that follow it, otherwise resembles it. The sixth vertebra (v.^) is 
quite free. 
The neural spines of the third and fourth vertebrae are confluent and together 
form a thin, high, compressed plate of bone which is not cleft or in any other way 
modified for the support of the interspinous bones of the dorsal fin. The spinous 
process of the fifth vertebra is quite distinct from the foregoing, and deeply cleft. 
The neural arches of the complex and fifth vertebrae are completely anchylosed 
together, but remain distinct from the ai’ch of the sixth. Three nutrient foramina 
are visible on the ventral surface of the complex centrum, two of which form a pair, 
and belong to the third vertebra, while the other is posterior to the former and 
median in position, and may therefore be regarded as belonging to the fourth vertebra. 
The transverse processes of the fourth vertebra are curiously modified, each being 
greatly expanded and having, except at its thick flat root, the anterior margin 
curved downwards and slightly backwards towards the posterior margin, which 
also curves slightly downwards to meet it In this way the transverse 
processes form on each side a spout- like protuberance from the side of the complex 
vertebra, enclosing a cavity or recess' which has complete bony walls in front, behind, 
and above, but is widely open towards the ventral surface. The anterior wall of each 
recess abruptly ceases at the flat root of the transverse process, and between its free 
inner margin and the lateral surface of the complex centrum there is a deep cleft, 
through which the tripus passes from its connection with the scaphium anteriorly to 
the air-bladder posteriorly. At the distal extremity of the process the anterior 
margin curves backwards towards the posterior margin to a somewhat greater extent 
than elsewhere, and slightly narrows the distal opening of the enclosed recess. The 
recess itself is somewhat oval in shape, being more expanded in the centre than it is 
distally or' proximally. The anterior and posterior walls are fairly thick, but the 
dorsal wall thins away over a sharply-defined concave oval area, and consequently 
becomes adapted to the convex dorsal wall of the contained air-sac. The anterior lip 
of the distal aperture of the recess is marked by a roughened concave surface, which 
becomes applied to the stem of the post-temporal in such a way as to form, ia con- 
junction with the latter, an articular socket for the proximal extremity of the clavicle. 
The transverse process of the fifth vertebra is very strongly developed, with a 
pointed free distal extremity and an expanded root, the anterior margin of which is 
partially confluent with the decurved posterior margin of its spout-like predecessor ; 
the process also helps to deepen posteriorly the recess in which each lateral air-sac is 
lodged. The two foramina visible on the ventral surface of the process transmit the 
rami ventrales of the fifth and sixth spinal nerves (6y;.n.“, sp.n,'^). 
Without forming an aortic canal, or even more than the merest trace of a groove 
