DIPNOI. 573 



The Teleosts include the great majority of living fishes, which 

 are classified in thirteen sub-orders and numerous families, e.g. 

 Clupeidae (herrings) ; Salmonidae (salmon, trout) ; Cyprinidse 

 (carps) ; Mursenidse (eels) ; Esocidae (pike) ; Gasterosteidae (stickle- 

 backs) ; Syngnathidae (pipe-fish and sea-horses) ; Gadidse (cod-fishes) ; 

 Percidae (perch) ; Scombridae (mackerels) ; Pleuronectidae (flat-fishes) ; 

 Cottidaa (bull-heads) ; Triglidse (gurnards) ; Lophiidae (anglers) ; 

 Tetrodontida; (globe-fishes). 



Sub-Class III. DIPNOI. "Mud-Fishes" 



Fishes with a lung the modified swim-bladder as well 

 as gills ; the paired fins are of the archipterygium type, with 

 a long segmented axis, sometimes bearing a series of lateral 

 pieces on each side, with overlapping cycloid scales, with 

 multicellular skin-glands, with a diphy cereal tail. The 

 notochord persists, and its sheath is unsegmented ; the skull 

 is autostylic and is largely a persistent chondrocranium with 

 the addition of some membrane bones; there are large 

 compound grinding teeth. The external nares are on the 

 ventral surface of the snout, or even within the upper lip, 

 and the arching over "of the nasal grooves leads to the 

 formation of separate internal nares. The heart is 

 incipiently three-chambered, containing mixed blood, with 

 a spiral conus arteriosus with numerous valves ; there is a 

 vein resembling the inferior vena cava of higher vertebrates. 

 There is a spiral valve in the intestine. The eggs are. large 

 and exhibit total unequal segmentation, as in Amphibians. 



The Dipnoi, whose name means double breathers, are 

 now represented by three genera Ceratodus^ from two 

 rivers of Queensland; Protopterus, from certain African 

 rivers, e.g. the Gambia ; and Lepidosiren, from the Amazons. 

 The wide distribution is noteworthy. 



They are very ancient forms, for Ceratodus existed in 

 Triassic and Jurassic times (though no post-Jurassic 

 remains are known). There were also undoubted Dipnoi 

 far back in Palaeozoic times, such as Dipterus and 

 Phaneropleuron of the Devonian, Ctenodus and Uronemus 

 of the Carboniferous. 



The living Dipnoi are probably the survivors of an 

 archaic group; in their teeth and autostylic skull they 

 resemble Holocephali ; in their fins and air-bladder they 



