CUVIER'S ORDERS. 



325 



small portions. They have no sutures in the cranium, 

 and the gelatinous substance which in other Fishes fills 

 the spaces between the vertebrae, and only extends from 

 one space to another by means of a small aperture, forms 

 in part of this group a long cord which traverses nearly 

 all the vertebrae without materially varying in its diam- 

 eter. This group includes three orders : 



Sturiones, or chondropterygians with free gills, as 

 Sturgeons ; 



Fig. 187. 



Sturgeon. 



Selachians, or chondropterygians with fixed gills, as 

 Sharks and Skates ; and 



Fig. 188. 



Skate. 



