DIPNOI. 511 



Besides these chief sub-orders, there are two sets of aberrant forms: 



(a) The sea horses, such as Hippocampus and Phyllopteryx, and the 



pipe fishes, such as Syngnathus^ are distinguished as Lopho- 

 branchii. The gills, instead of being rows of filaments, are 

 tufts of rounded lobes ; the gill cover is a simple plate, leaving 

 a small aperture ; the skin is more or less protected by large 

 dermal plates ; the toothless mouth is at the end of a prolonged 

 snout. 



(b] The globe fishes, such as Tetrodon and Diodon, the trunk fishes 



Ostracion, the sun fish Orthagoriscus, and others, are distin- 

 guished as Plectognathi. The body is globular or compressed 

 sideways ; the skin bears bony scutes or spines, or is naked ; 

 the skeleton is incompletely ossified, and the vertebrae are few ; 

 the bones of the upper jaw are more or less fused ; the pelvic 

 fins are absent or reduced to spines ; the gills are comb-like ; 

 the swim bladder has no duct. 



It is likely that some of the loosely-built deep-sea fishes, such as the 

 pelican fish Eurypharynx, are not referable to the orders usually recog- 

 nised. 



Order IV. DIPNOI " Mud Fishes." 



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 or a closely 

 allied form has lived on from Mesozoic times, and there 

 were also undoubted Dipnoi far back in Palaeozoic times, 

 such as Dipterus and Phaneropleuron of the Devonian, 

 Ctenodus and Uronemus of the Carboniferous. According 

 to some, the remarkable Devonian Coccosteidae are also to 

 be considered as an aberrant group of Dipnoi. 



Prof. W. N. Parker regards them as "the isolated sur- 

 vivors of an exceedingly ancient group, which was probably 

 nearly allied to the ancestors of existing Amphibians and 

 Fishes, more particularly Elasmobranchs, though the Ganoid 

 stock most likely arose not far off." 



Were it not for the disadvantage of multiplying classes, 

 one would be inclined to place them between Pisces, which 

 they resemble in having cycloid scales, paired fins, a spiral 

 valve, &c , and Amphibia, which they approach in having 

 lungs, an incipiently three-chambered heart, a vena cava, 

 a pulmonary vein, posterior nares, and multicellular skin 

 glands. 



