PISCES 



373 



392. Classification of Fishes. 



Subclass I. Elasmobranchii (Sharks, Dog-fishes, Rays, Skates). — This group 

 is often recognized as a separate class. Marine fishes with essentially cartilaginous 

 skeleton; no operculum or gill-cover; mouth on the ventral surface of the head; 

 heterocercal tail; external skeleton of placoid scales; spiral valve in the intestine; 

 no air bladder. The elasmobranchs are regarded by some as being the nearest 

 present relatives of the primitive fishes. They occur most abundantly and are 

 larger individually in warm seas. They are powerful swimmers as befits carnivor- 

 ous, preying animals. They feed on Crustacea, MoUusca, and fish. 

 Subclass II. Teleostomi. 



Order I Crossopterygii. — Vertebral column well ossified; tail diphycercal; 

 pelvic fins, when present, abdominal. Polypterus and Calamoichthys 

 include the only existing representatives. 

 Order 2. Chondrostei. — Skeleton largely cartilaginous; ganoid scales; tail 

 heterocercal; pelvic fins abdominal. Paddle-fishes and sturgeons. 

 Largely fresh-water forms. Feed upon small organisms. 



Pig. 187. 



Fig. 187. Long-eared Sunfish {Le/)omi5 (iMri/;(5}. Adult. Photo from life by Dr. R. W. Shufeldt. 



Order 3. Holostei. — Skeleton bony; scales ganoid or cycloid; tail diphycercal 

 or homocercal; ventral fins abdominal. Gar-pikes and bow-fins. Preda- 

 tory on other fish, crayfish, moUusks, etc. 

 Order 4. Teleostei. — Skeleton bony; tail usually homocercal; scales cycloid 

 or ctenoid. Carp, suckers, catfish, white fish, trout, pike, etc. This 

 order contains the majority of living species of bony fishes. Many of 

 the species are very important commercially. Pood habits varied. 

 Subclass III. Dipnoi (Lung-fishes). — Fishes with a persistent notochord and 

 the internal skeleton incompletely ossified; soft cycloid scales; spiral valve in the 

 intestine, the swim-bladder used as a lung, the auricle partly separated into two 

 chambers, paired appendages with a central axis producing a flapper rather than 

 a fin (Fig. 177). There are only three genera of living representatives, but these 

 are especially interesting to the zoologist from the fact that they may represent 

 the division of fishes from which the air-breathing vertebrates sprang. One genus 

 (Neoceratodus) is found in the rivers of Queensland; the second (Prolopterus) in 

 the rivers of southern Africa, and a third (Lepidosiren) in the Amazon in South 

 America. No marine forms are known. From fossil remains it is evident that 

 the ancestors of the present lung-fishes were very much more widely distributed. 



