122 L UNG-FISHES 



perfect evidence is discovered as to their kinship and the 

 lines of their descent. 



In the history of fishes, Dipnoans are known to have been 

 early a dominant group. In some regards, one of their 

 ancient forms bore many resemblances to the Pleura- 

 canthid shark, which, although known at present only in 

 a later period, may well have been its contemporary. But 

 the range in the forms of Dipnoans occurring in the early 

 Palaeozoic indicates the remote antiquity of their origin. 

 They had even then evolved exoskeletal characters which 

 are scarcely less specialized than those of existing forms. 



Fig. 126. — A restoration of the Devonian lung-fish, Phaneropleuron. x \. 



Dipterus, of the Old Red Sandstone (Fig. 123), had a 

 complete body armouring of cycloidal scales, a head roofing 

 of dermal plates (Fig. 124), and well-calcified jaw rims 

 (Figs. 124, 125, 12$ A). Its fin rays were dermal in 

 structure, its paired fins were archipterygial, its tail and 

 its dorsal fins separate and lobate. Its mucous canals had 

 become elaborately adapted to the body scales (lateral line, 

 Fig. 123) and head plates, piercing the latter with minute 

 pores, as in Figs. 65, 66. Anterior and posterior nares are 

 indicated under the rim of the upper jaw (Fig. 125, 1-2). 

 Marginal teeth have disappeared ; a pair of elaborate dental 

 plates on the mouth roof (palatine) are apposed by a simi- 

 lar pair in the hinder part of the mandible (splenial). 



The Carboniferous Cteiiodus was a nearly allied form. 



Another Devonian lung-fish, PJianeroplein'on (Fig. 126), 



