68 



IOWA GEOLOGICAL SURVEY 



SCHEME OF SUBCLASSES AND ORDERS OF THE CLASS PISCES 



Stages in Evolution of 

 Paired Fins 



Hyostylic Fishes 



ELASMO- 

 BRANCHII 



Autostylic Fishes 



TELEOSTOMI 'HOLOCEPHALI DIPNOI 



1. Supports as parallel 

 rods 



2. Archipterygium of 

 Gegenbaur (elon- 

 gate or abbreviate) 



3. Pectorals di- or tri- 

 basal; pelvics ab- 

 breviate 



4. Basal cartilages 

 small or rudiment- 

 ary 



PLEUROPTER- 

 YGII 



(early Palaeo- 

 zoic) 



ICHTHYOTOMI 



(late Palaeo- 

 zoic) 



SELACHII 

 (late Palaeo- 

 zoic to Re- 

 cent) 



ACANTHODII 



(Palaeozoic) 



[Unknown] [Unknown] [Unknown] 



CROSSOPTER- [Unknown] 

 YGII 



(Palaeozoic 

 and M e s o- 

 zoic) 



CROSSOPTER- CHIMAERO- 

 YGII IDEI 



(early Meso- 

 zoic to Re- 

 cent) 



(Recent) 



ACANTHOP- I [Unknown] 



TERYGII J L 



(Palaeozoic to 

 Recent) 



SIRENOIDEI 

 (Palaeozoic to 

 Recent) 



[Unknown] 



ARTHRODIRA 



(early Palaeo- 

 zoic) 



We will conclude our remarks on the subject of classification 

 by presenting the following outline scheme, which shows the 

 arrangement of the higher divisions of fishes and fish-like ver- 

 tebrates that seems to accord best with the present state of our 

 knowledge, and is sanctioned by competent authorities. 



