652 DR JAMES STARK ON THE EXISTENCE OF AN OSSEOUS STRUCTURE 



vertebrae are extremely solid, and bear a very close resemblance to those of the 

 saw-fish. Each vertebra is formed of two flat shallow saucer-shaped discs of 

 bone, united to each other by four very broad osseous pillars which extend from 

 the margins of the cups to the centre of the vertebra. The margins of the cups pro- 

 ject somewhat beyond those broad supports which form the body of the vertebra. 

 These strengthened portions are on the abdominal and dorsal, and two lateral sur- 

 faces : the intermediate portions, however, are hollowed out, and form square 

 cavities filled with cartilaginous matter, which extend nearly to the centre of the 

 vertebra. (Fig. 12 represents one of the dorsal vertebrae. Kg. 12. 



A the anterior or abdominal supporting plate ; B the lateral hol- 

 low ; C the flat articular surface.) The examination of the ar- 

 ticular surface shews that, in these vertebrae, the osseous mat- 

 ter of the cups is deposited in concentric layers, as in all the other cartilaginous 

 fishes ; but not having had an opportunity of making a section of the bone, I am 

 unable to speak of the peculiar disposition of the supporting plates which form 

 the body of the vertebra. 



Two other orders of fishes are arranged under the chondropterygious sub- 

 class, viz., the Cydostomi and Sturiones. These two orders, however, differ in 

 many essential points from those cartilaginous fishes the structure of whose 

 vertebrae we have been considering : For, while the Plagiastomi, which include 

 all the Sharks and Rays, possess, generally speaking, an organization of a higher 

 order than that of the osseous fishes, the Sturiones, but especially the Cydostomi, 

 possess an organization of an inferior order, an organization which renders them 

 in some measure the connecting link between the more highly organized Mol- 

 lusca and the Fishes. 



This circumstance is accordingly not only seen in their digestive, generative, 

 and nervous development, but extends, in a remarkable degree, to what ought to 

 constitute their internal skeleton. In the Sturiones, or Sturgeons, the spinal 

 column is formed of a continuous purely cartilaginous tube, divided at intervals 

 into regular pieces, but with no osseous matter whatever in that part which 

 ought to constitute the bodies of the vertebrae. Mr JONES, therefore, must have 

 made some mistake in the passage above quoted, in instancing the spinal column 

 of the Sturgeon as one in which osseous matter is encroaching on the cartila- 

 ginous matter of the bodies of the vertebrae ; while Dr GRANT and CUVIER are 

 strictly correct in describing the vertebral column of the Sturgeon as consisting 

 of soft transparent cartilage, having earthy salts only deposited on the laminae. 



It is to be recollected that the Sturgeon is one of those animals in which the 

 external skeleton is very strongly developed ; and the different rows of osseous 

 plates or shields are so placed as mutually to support one another, and thus keep 

 the body extended, even although no osseous structure is developed in the spinal 

 column. 



