DIPNOI. 553 



The Teleosts include the great majority of living fishes, which 

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

 Clupeidre (herrings) ; Salmonidie (salmon, trout) ; Cyprinidre 

 (carps) ; Muraenidre (eels) ; Esocidse (pike) ; Gasterosteidre (stickle- 

 backs) ; Syngnathidre (pipe-fish and sea-horses) ; Gadidae (cod-fishes) ; 

 Percidre (perch) ; Scombridae (mackerels) ; Pleuronectidre (flat-fishes) ; 

 Collide (bull-heads) ; Triglidre (gurnards) ; Lophiido: (anglers) ; 

 Tetrodontidiu (globe-fishes). 



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



Fishes with a lung the modified swim-bladder as well 

 as gills, with internal as well as external nares, with a. long 

 segmented axis in the paired fins, sometimes bearing a 

 series of lateral pieces on each side (archipterygia), with 

 overlapping cycloid scales, with multicellular skin-glands, 

 with a diphycercal tail. The notochord persists and its 

 sheath is unsegmented ; the skull is cartilaginous and 

 autostylic ; there are large grinding compound teeth. 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, Ctcnodus 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 

 recall Crossopterygii ; in their cartilaginous skeleton and 

 persistent notochord they are primitive ; in their internal 

 nares, lung, heart, inferior vena cava, multicellular skin- 

 glands, and eggs they approach Amphibians. 



