39 



towards the posterior end. Behind the last of these vertebras are bones which 

 terminate the vertebral column : these consist of a central fan-shaped bone, the anterior 

 end of which is shaped like the anterior part of a vertebra, and is connected with the 

 posterior face of the last vertebra by membrane, and two other flat bones, dorsal and 

 ventral to the central one. The central bone expands posteriorly in the median plane 

 of the body into a triangular plate, which is partially divided by radiating furrows into 

 a number of rods. These rods are known from their development to represent the 

 ventral spines of a number of vertebras fused together into the fan-shaped bone. The 

 flat bones dorsal and ventral to the fan-shaped bone are similarly divided, but are much 

 narrower : these are obviously also dorsal and ventral spines of the more anterior of 

 the vertebras represented by the fan -shaped bone. The rays of the caudal fin articulate 

 with the ends of these rods (PI. X). 



The ten anterior vertebras differ considerably from the posterior 40. All of them 

 have dorsal spines except the first : these spines resemble those of the vertebras already 

 described, except that they become thicker and shorter and more inclined forwards 

 towards the anterior end. The dorsal spine of the second vertebra leans forward so 

 much that it is in contact with the posterior face of the skull. The fifth to the tenth 

 vertebras have ventral spines also, but these are of rapidly decreasing length from the 

 tenth to the fifth, and are also much inclined backwards. The first four vertebras 

 have no ventral spines. The second to the eighth vertebras, seven in all, bear very 

 slender short ribs which are not processes of the vertebrae, but separate bones which 

 articulate with the centra at the upper side of their lateral ridges. The first vertebra 

 is rudimentary : its centrum is very narrow antero-posteriorly, and it has two small 

 dorsal processes which lie along the front edge of the base of the dorsal processes of 

 the second vertebra, but do not unite to form a spine. 



The centra of all these vertebras are united together in the following way. The 

 posterior face of each vertebra has, as we have seen, a conical depression or cavity : the 

 rim of this cavity is united firmly to the corresponding rim on the anterior face of the 

 succeeding vertebra by strong tough membrane, which is continued over the surface of 

 the vertebras, which is in fact part of the periosteal membrane. The closed cavities 

 formed by the juxtaposition of the conical depressions are filled by a firm elastic 

 gelatinous tissue which forms so many elastic pads between the centra. The connection 

 of the ex-occipital bone of the skull with the first vertebra is of the same kind as that 

 between adjacent vertebras. 



Closely connected with the vertebral column is the system of bones which forms the 

 framework and support of the median fins. Placed like the vertebral spines in the 

 median plane of the body and extending outwards from the ends of those spines are a 

 great number of bones shaped somewhat like paddles. These are the interspinous bones. 

 Each consists of a long slender shaft and a broad head expanded in the median plane 

 They are placed with their shafts towards the vertebras, their heads projecting 

 outwards. The ends of the shafts lie alongside the ends of the spines of the vertebras 



