Fishes have been classified in different ways from time to time. Earliest classification was proposed by J. Muller (1844) who proposed 7 subclasses in class Pisces.
Muller’s classification (1844)
Class PISCES
Subclasses: DIPNOI (Lung fishes); TELEOSTEI (Bony fishes); GANOIDEI (Polypterus, Amia, Lepidosteus); ELASMOBRANCHI (Cartilagenous fishes); MARSIPOBRANCHI (Cyclostomes); LEPTOCARDII (Amphioxus).
Berg’s classification (1940)
Series PISCES
Class PLACODERMI (Pterichthyes; Coccostei; Acanthodi)
Class ELASMOBRANCHI (Cartilagenous fishes)
Class HOLOCEPHALI (Chimaera)
Class DIPNOI (Lung fishes)
Class TELEOSTOMI
Subclass Crossopterygii (Latimeria)
Subclass Actinopterygii (Bony fishes).
Romer’s classification (1959)
Series PISCES
Class PLACODERMI (Pterichthyes; Coccostei; Acanthodi)
Class CHONDRICHTHYES
Subclass Elasmobranchi (Sharks, dogfishes, skates, rays)
Subclass Holocephali (Chimaera)
Class OSTEICHTHYES
Subclass Sarcopterygii
Order CROSSOPTERYGII (Latimeria)
Order DIPNOI (Lungfishes)
Subclass Actinopterygii (All bony fishes)
Parker & Haswell’s classification (1967)
Series PISCES
Class PLACODERMI (6 Subclasses)
Class CHONDRICHTHYS
Subclass Selachi (Sharks and Rays)
Subclass Bradyodonti (=Holocephali) (Chimaera)
Class OSTEICHTHYS
Subclass Crossopterygii (Latimeria)
Subclass Actinopterygii (Rayfin bony fishes)
Class DIPNOI (= CHOANICHTHYES) (Lung fishes)
COMBINED CLASSIFICATION (After Romer, 1959 and Parker & Haswell,1967)
Series PISCES
Class PLACODERMI (6 Subclasses) Extinct.
Class CHONDRICHTHYS
Subclass Elasmobranchi
Marine with placoid scales; pelvic fins modified as claspers; tail heterocercal; mouth ventral; cloaca present; spiracle present; operculum present.
Superorder CLADOSELACHI (Extinct Sharks)
Paired fins biserial with a median axis; vertebra not differentiated; tail fin variable.
Superorder SELACHI (Modern cartilaginous fishes)
Paired fins finfold type; tail fin heterocercal; males with claspers; 5 pairs of gill slits; operculum absent; No air bladder; placoid scales present.
Order Pleurotremata (Shaks and Dogfishes), 400 species.
Body cylindrical; gill slits laterally placed; pectoral fins small; eyes laterally placed.
Order Hypotremata (Skates and Rays), 450 species.
Body dorsoventrally flattened; gill slits ventrally placed; pectoral fins large and fused on the lateral margins; bottom dwellers.
Subclass Holocephali (Chimaera), 30 species.
Four pairs of gills; notochord persists; teeth modified as crushing plates; mouth with lips; holostylic jaw suspension; tail whip-like; operculum present; spiracles absent but a single nostril present; skin naked with scattered placoid scales.
Class Osteichthys
Subclass Crossopterygii (Latimeria), 2 species.
Internal nares present; fins lobed except the first dorsal fin; scales with cosmine layer; air bladder lung-like; jaw suspension autostylic; notochord persists and vertebrae unossified;
Subclass Actinopterygii (Rayfin bony fishes), 22,000 species.
Fins with fin rays; tail fin homocercal; scales cycloid and ctenoid; internal nares absent; air bladder present but no lungs; jaw suspension methystylic.
Superorder CHONDROSTEI (35 species)
Skeleton partly cartilaginous; ganoid scales present; two internal nostrils present; air bladder two-lobed and ventral.
Order Polypteriformes (Polypterus), 10 species.
Order Acipenseriformes (Acipenser; Polyodon), 25 species
Superorder HOLOSTEI (8 species)
Skeleton partly cartilaginous; teeth sharp; ganoid scales on body; air bladder lung-like; tail fin homocercal; vertebrae opisthocoelous.
Order Amiiformes (Amia calva)
Order Semiontiformes (Lepidosteus), 7 species.
Superorder TELEOSTEI (20,000 species)
Skeleton bony; tail homocercal; scales cycloid and ctenoid.
Order Clupeiformes (Herrings; Salmons, Sardines)
Order Scopeliformes (Bombay duck)
Order Cypriniformes (=Ostariophysi) (Labeo, Catla, Mystus, Clarias, Wallago, Heteropneustes)
Order Anguilliformes (Eels)
Order Beloniformes (Flying fishes)
Order Syngnathiformes (Sea horse and Pipe fish)
Order Mastacembeliformes (Eel-like Mastacembelus)
Order Ophiocephaliformes (Ophiocephalus)
Order Symbranchiformes (Eel-like Amphipnous)
Order Perciformes (Lates; Perca)
Order Scorpaeniformes (Scorpion fish)
Order Pleuronectiformes (Flat fishes)
Order Echeneiformes (Sucker fish)
Order Tetraodontiformes (Tetrodon; Diodon; Ostracion)
Order Lophiiformes (Angler fishes)