The Osteichthyes or Bony Fishes 



Most of our modern fresh-water fishes are characterized by the 

 presence of bone, which distinguishes them from cartilaginous fishes, 

 such as the sharks and their relatives. These latter are characterized 

 by primitive skeletons entirely of cartilage and by uncovered gill-clefts. 

 The cartilaginous fishes are mostly marine; none are found in the fresh 

 waters of North America. 



Most fresh-water fishes are highly developed forms. There are a few 

 primitive types, Chondrostei and Holostei, in which very little bone is 

 present, such as the sturgeons, gars, paddlefish, and dogfishes. These 

 forms retain many characteristics of the ancestors of the more highly 

 developed modern forms, the Teleostei, which may be called the bony 

 fishes. 



The bony fishes as well as the cartilaginous fishes (Chondrichthyes) 

 are differentiated by the development of upper and lower jaws and 

 two pairs of appendages, or fins. Scales of some type are usually pres- 

 ent, although in some fishes they may be lost. Teeth are usually pres- 

 ent, but a few species have lost their teeth or have only vestigial ones. 



The skeleton of the Teleostei is fully formed and is usually of bone, 

 although some of the more primitive do not have the bones completely 

 ossified, and in a few the bones may remain partly or wholly of car- 

 tilage. The gill-clefts are always covered by an opercular flap (see 

 Diagram 1) . 



