IN THE VERTEBRAL COLUMN OF CARTILAGINOUS FISHES. 947 



In the Rays, then, the vertebral column, far from being solely formed of car- 

 tilage, consists essentially of true bone, surrounded, it is true, by cartilaginous 

 matter, and having all its processes formed of cartilage, with its attendant 

 covering of calcareous granules, but still consisting essentially of the very same 

 structure as the vertebrae of osseous fishes. 



Each vertebra consists internally of two cup-shaped osseous pieces united 

 by their narrow bases, or of two truncated cones united by their truncated 

 apices, so that each vertebral bone resembles a wooden egg-cup, or an hour-glass. 

 Towards the extremity of the tail of the Rays, these hour-glass-shaped vertebras 

 are nearly simple ; or the point of union between the two cups that part which 

 forms the body of the vertebra is swollen out into a bulging ring, as at fig. 1, a. 

 In some Rays this form of vertebra is often alternated with the simple double 

 cup-shaped one. The bodies of all these vertebrae are perforated by an aperture 

 so minute as scarcely to admit the point of the finest needle. (Fig. 1.) 



As the body of the animal is approached, these double cups receive 

 strengthening pillars or plates of osseous matter, first on Kg v 



two opposite sides (Fig. 2), and soon on four sides (Fig. ^ ^ 

 3), so as perfectly to support the enlarged size of the ver- W )\ J\ 

 tebrse in all violent motions of the animal. These solid 

 strengthening pillars or plates are placed perpendicularly to 

 the outer surface of the vertebrae, and are always strongest 

 and broadest on the upper or dorsal surface, and on the lower 

 or abdominal surface, and fill up more or less of the angular Fig. 3. 



space between the two cups of each vertebra (Fig. 3). The 

 upper or dorsal plate is generally single, and, in all the larger 

 vertebrae, extends forward to the edge of the cup of the articu- 

 lar surface of the vertebra (Fig. 3, b). The plate on the anterior or abdominal sur- 

 face of the vertebra is either double in the larger bones (Fig. 4, a), or consists of a 

 broad, somewhat grooved, plate in the centre, opening out into two plates or sup- 

 porting bases, as it reaches the edge of the vertebral cup (Fig. Fig 4 K 6 

 5, a). In all the larger vertebrae, the anterior plate or plates 

 extend forwards to the outer margin of the cup-shaped arti- 

 cular surfaces of the vertebrae, and thus fill up the whole 

 angular space. It is otherwise, however, with the lateral 

 columns. They rarely fill up the whole angular space left by the union of the 

 articular cups of each vertebra, but only about a third or half of that space (Figs. 

 4 and 5, b b b). In the upper dorsal vertebrae, however, these lateral pillars are 

 often double, and advance forwards to the edge of the articulating cups. The 

 central aperture which pierces the body of the largest vertebra, is not larger 

 than that of the smallest vertebra scarcely admitting the point of the finest 

 needle. 



VOL. XV. PART IV. 8 M 



