220 



BRANCH CHORDATA 



The strange development or change of the flounder from a sym- 

 metric to a "flat" fish demonstrates the principle that special 

 habits of life result in special modifications of structure which 

 fit the animal for those habits. 



The lung-fishes (Dipnoi) show many advancements toward 

 the air-breathing conditions, such as the swim-bladder used as 

 a lung and the partly separated auricle, and the flipper with a 

 central axis rather than a fin, so that zoologists are led to be- 

 lieve that they may represent the division of fishes from which 

 the amphibians sprang. 



Classification. — 



Sub-phylum Vertebrata or Craniata. 



Class I. Cyclostom'ata. 



Order I. Petromyzon'tes. 

 Order II. Myxinoi'dei. 



Class II. Pis'ces. 



Sub-class I. Elasmobran'chii. 

 Order I. Cladosela'chea. 



Order II. Pleuracan'thea. 

 Order III. Scla'chii. 



Sub-class II. Holoceph'ali. 



Sub-class III. Dip'noi. 



Order I. Monopneu'mona. 

 Order II. Dipneu'mona. 



Lamprey, hag-fishes. 



Lamprey. 



Hag-fishes. 



Extinct shark-like forms. 



Cladoselache. 

 Extinct Pleuracanths. 

 Extinct forms and all the 



living Elasmobranchs, 



as sharks and rays. 

 Three genera of Chimcc- 



ridce. 



Ceratodus. 



Protopterus and Lepidosi- 



Sub-class IV. Teleos'tomi. 



Order I. Crossopteryg'ii. 



Order II. Chondros'tei. 



Order III. Holos'tei. 



Order IV. Teleos'tei. 



