650 DR JAMES STARK ON THE EXISTENCE OF AN OSSEOUS STRUCTURE 



The two cup-shaped bodies of which each vertehra is composed, are, on their 

 outer margin, not perfectly circular, but of an irregular, somewhat hexagonal, 

 form, and consist of a truly osseous structure, differing little, excepting in in- 

 creased density, from the vertebral bones of osseous fishes. The osseous matter 

 is, as in osseous fishes, deposited in concentric rings. The mode in which these 

 cups are strengthened differs, however, as might be expected, from that observed 

 in the Rays. The motions of this family of fishes are more quick and varied 

 than those of the rays ; and no class, perhaps (Eels alone excepted), possesses a 

 greater amount of flexibility in their spinal column than the Squalidse or Sharks. 



The double cup-shaped vertebrae, then, are not supported by pillars or plates 

 which fill up the angle formed by the sloping sides of each cup, as it is in the 

 Rays, but the supporting plates are wholly external, and consist of six flat plates 

 of osseous matter, stretching from the outer edge of the one cup to the outer edge 

 of the other of the same vertebra. (Fig. 9, b). The breadth of each plate corresponds 

 to that of the hexagonal side of the cup which it supports. The anterior or ab- 

 dominal side being much the narrowest, possesses the narrowest supporting 

 plate ; but this plate is in general grooved. The other plates are of nearly 

 equal breadth, and almost touch each other by their margins. In the recent 

 animal, however, the margins are prolonged into cartilaginous processes, covered 

 with calcareous particles, and form the various vertebral laminae, processeses 

 and canals, remarked on the spinal column. The free space between the osseous 

 cups and the supporting plates is filled up with tough cartilage a cartilage re- 

 moveable by boiling. (To exhibit this, a section is re- Fig. 9. 



presented at Fig. 9, c.) <^ 



Notwithstanding the somewhat hexagonal form of >\ 

 the external margin of the cups of each vertebra, the 

 articular surfaces, or rather the internal cavity, of the cup is nearly circular. 



The same general structure was found present in the Spinax Acanthias, the 

 Picked Dog-fish, in the Scyttium Canicula, the Spotted Dog-fish, the Scyllium Ca- 

 tulus, the Common Dog-fish, and the Galeus vulgaris, the Common Tope. It is 

 reasonable, therefore, to infer, that the same structure will be found present in 

 all the allied species. 



In the other genera of the great family of Squalidse or Sharks, the osseous 

 portion of the vertebral column presents various modifications of structure. In 

 the Selache maximus, or Basking Shark, the vertebrae are of much larger dimen- 

 sions, in proportion to the size of the animal, than in the genera above alluded 

 to. This is easily accounted for when the structure of these vertebrae is exa- 

 mined. The osseous matter is not, as in the dog-fish, deposited as a central 

 nucleus of stony hardness, but is deposited in concentric plates, each of which is 

 separated from the adjoining one by a layer of cartilage. From the centre to 

 the circumference of the vertebrae, therefore, there is presented alternate layers 



