222 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
organ by Day (9) in the following terms : — “Air-vessel small and placed transversely 
across the body of the anterior vertebra, where there is a groove to receive its 
posterior surface ; anteriorly, it has a thick strong fibrous covering. There is a low 
osseous process from the vertebra giving it protection laterally. The air-vessel itself 
consists of two small oval portions, having a median connecting tube, and this lateral 
part is surrounded by osseous or strong fibrous walls” [he. cit., p. 712). As the 
species may attain a length of four feet, it is obvious that our specimen was a very 
immature one, but this fact alone can hardly account for the marked discrepancy 
between the conflicting statements of these writers, and the results of our own 
dissections as stated below. 
The body of the first vertebra is a thin discoidal bone with but slightly concave 
anterior and posterior surfaces, and quite distinct from the complex. centrum and basi- 
occipital. The complex and fifth vertebrm alone are rigidly connected together by 
the superficial ossifications and the partial anchylosis of their neural arches. The 
sixth vertebra is quite free. The spines of the third and fourth vertebrae form a thin 
vertical plate of bone with thickened and produced anterior and posterior margins. 
The anterior margin is inclined forwards and bifid ; occupying the cleft, and connect- 
ing the spine with the supraoccipital and exoccipital bones of the skull, is a consider- 
able amount of intercalated cartilage. The posterior margin is inclined obliquely 
backwards and deeply cleft for the support of certain of the anterior interspinous 
bones of the dorsal fin. 
The flat roof of the transverse process of the fourth vertebra splits distally into an 
anterior and a posterior division, tlie former (fig. 97, curving downwards and 
supporting the anterior wall of the bladder, and in front articulating by a well- 
marked facet with the post-temporal so as to complete the socket for the clavicle. 
The slender transverse processes of the fifth and sixth vertebrm t.p}) are 
widely distinct from the foregoing ; proximally, they are slightly confluent but their 
distal extremities are quite free. The sixth vertebra carries the first pair of ribs (r^)- 
The ventro- lateral margins of the complex and fifth centra are greatly thickened by 
superficial ossified deposit, which is prolonged ventrally on each side to form the lateral 
walls of a median aortic groove. The dorsal lamina (fig. 98, d.l.) is represented by a 
slender spicule of bone, which on each side becomes detached from the ventral surface 
of the transverse process of the fourth vertebra, close to the complex centrum, and, 
after a slightly oblique course forwards and downwards, ventrad to the posterior 
cardinal vein of its side, terminates in a free anterior or ventral extremity which 
receives the insertion of the radial fibres of the tripus. 
The air-bladder (fig. 97) has a very unusual structure. In shape it is somewhat 
reniform, with its long axis transversely disposed and its anterior wall very slightly 
concave, while its posterior contour is strongly convex.* The lateral portions of the 
anterior wall are supported by the deflected anterior divisions (i.p.%.) of the modified 
* The dimeusions of the bladder in our .specimen were, width 10 mm., length 9 mm. 
