INTRODUCTORY 5 



(Cyclostomes) , with their pouch-like gills and mouths devoid of 

 biting jaws, resemble some of the true fishes to a certain super- 

 ficial extent in outward form, in habits, and in their general 

 manner of breathing, and may well be regarded as "fishes" 





Fig. 3. — EXTINCT REPTILE OF FISH-LIKE FORM. 



Restoration of the Ichthyosaurus. Much reduced. 



in the popular sense. Actually, the two groups of animals are 

 separated by characters just as fundamental as those which 

 divide all the other fishes from the amphibians, and the Cyclo- 

 stomes must, therefore, rank as a separate class {cf. p. 344). 

 The same is true to a lesser extent of the Selachians, a group 

 including the Sharks, Rays, and Chimaeras, which have been 



