28 VERTEBRATE ANIMALS OF THE UNITED STATES 



Subclass 2. Pisces. — The true fishes. Aquatic vertebrates with 

 gills which are attached to bony or cartilaginous gill-arches; with true 

 jaws; with median fins and usually two pairs of paired fins; in most cases 

 with scales; with paired nostrils: 3 divisions. 



Key to the Divisions of Pisces 



ai Skeleton cartilaginous; skull without membrane bones; sharks; 



skates; (all marine, not included in this book) i. Selachii. 



Z.2 Skeleton in most cases bony; skull with membrane bones. 



bi Paired fins each with a segmented cartilaginous axis; respiration 

 by gills and lungs (no American species and hence not 



included in this book) 2. Dipnoi. 



hi Fins supported by fin-rays and without central axes 3. Teleostomi. 



Division 3. Teleostomi. — Skeleton usually bony; skull covered 

 with membrane bones separated by sutures ; a single gill opening on each 

 side, the gills being supported by 4 pairs of arches in most cases, and 

 covered by the opercles; air bladder mostly present: 2 series. 



Key to the Series of Teleostomi 



ai Skeleton cartilaginous or bony i. Ganoidel (p. 28). 



a2 Skeleton bony 2. Teleostei (p. 33). 



Series I. Ganoidei. — The ganoid fishes. Fishes with either car- 

 tilaginous or bony skeleton; with heterocercal or homocercal caudal fin; 

 with either ganoid or cycloid scales or bony dermal plates; with an air 

 bladder provided with an open duct connecting it with the pharynx or 

 the oesophagus; with an arterial bulb with several pairs of valves; and 

 with an intestinal spiral valve: 2 American orders, with about 13 Ameri- 

 can species. 



Key to the American Orders of Ganoidei 



ai Ggjioids without scales, with the body either naked or 



with bony plates; skeleton cartilaginous i. Chondrostei (p. 28). 



a2 Ganoids with the body covered either with ganoid or 



cycloid scales; skeleton bony 2. Holostei (p. 31). 



Order i. Chondrostei. — Cartilaginous ganoids. Fish of large 

 size and with the skeleton cartilaginous; notochord persistent; branch- 

 iostegal rays weak or wanting; mouth ventral beneath a long snout; 

 body naked or with bony plates; tail heterocercal: 2 suborders. 



Key to These Suborders 



ai Body naked i- Selachostomi. 



a2 Body more or less covered with bony plates 2. Glaniostomi. 



