IN THE VERTEBRAL COLUMN OF CARTILAGINOUS FISHES. (}49 



column permits a small amount of flexibility in the body of each vertebra ; and 

 the jointed external lateral supports, while they prevent that flexion being 

 carried too far, in consequence of the square particles of calcareous matter sup- 

 porting each other, tend, by their partly cartilaginous nature, to restraighten the 

 vertebrae. 



As already remarked, the above described osseous structure was found pre- 

 sent in the vertebral column of the Common Skate, Thornback Ray, Long-nosed 

 Ray, and Starry Ray ; so that I have no doubt it is present in all this order of 

 cartilaginous fishes. As might be expected, the minute disposition of these parts 

 varies a little in each species, yet not so much as to deviate in any material point 

 from the above description. 



Before leaving the Rays it may be as well to point out the mode in which 

 these osseous portions of the vertebrae are united to the cartilaginous tube which 

 occupies the place of the cervical vertebrae (if we may so speak) in these animals. 

 All writers, as above noticed, simply describe this portion of the spine as a car- 

 tilaginous tube. But it is instructive to remark, that this cartilaginous tube, at 

 its lowest part, contains the solid osseous bodies of several vertebra?, united to 

 each other by the same regular intervertebral Fi e- s - 



ligamentary apparatus, as the free vertebrae. 

 These osseous portions, however, as they approach 

 the head, gradually diminish in size, and at last 

 terminate in a minute cup, which is regularly ar- 

 ticulated to the lower vertebra, but superiorly 

 gives place to the cartilaginous mass which supplies the place of vertebrae in the 

 upper part of the spine. (Fig. 8.) 



Being anxious to ascertain whether another great family of cartilaginous 

 fishes contained osseous nuclei in their vertebral column, I procured a few species 

 of Dog-fish, animals belonging to the family of Squalidse or Sharks, as this family 

 had been characterised by Baron CUVIEB as one of those whose skeletons con- 

 tained no true bone, but consisted of cartilage, soft within, and with an intersper- 

 sion of calcareous granules on the surface only. 



The internal nucleus of the vertebrae of these animals was found to consist 

 of a truly osseous basis, of much the same general form as that of the Rays. 

 Each vertebra consisted essentially of two cup-shaped bodies, or hollow truncated 

 cones, united by their truncated apices, and resembled wooden egg-cups still more 

 closely than the vertebras of the Rays. (Fig. 9, a). When perfectly cleaned, it was 

 seen that the cup on the one extremity of the vertebra communicated by a large 

 rounded aperture with that on the opposite extremity, as had been remarked in 

 1809 by Sir EVEHARD HOME, when describing the intervertebral structure of the 

 Squalus maximus, or Basking Shark. The aperture is smaller in the more minute 

 vertebrae, and of larger diameter in the larger vertebra?. 



