DEVONIAN FISHES OF IOWA 157 



Subclass DIPNEUSTI. 



(Dipnoans or Lung-Fishes.)* 



Fishes with partially ossified skeleton, numerous membrane 

 or dermal bones, and persistent notochord; skull autostylic; 

 dentition confined to inner bones of the mouth ; premaxillae and 

 maxillae absent; gill-clefts feebly separated, opening into a 

 cavity protected by two opercular plates ; paired fins archiptery- 

 gial or reduced; tail diphycercal or heterocercal, median fins 

 often subdivided; exoskeleton consisting of true bony tissue; 

 sensory canals well developed; nostrils inferior; claspers ab- 

 sent ; a cloaca present, air-bladder single or paired, functioning 

 as a lung. 



The few existing Dipnoan species, comprised by the fresh- 

 water genera Neoceratodus, Protopterus and Lepidosiren, form 

 a well-nigh inappreciable remnant of a once flourishing and 

 highly diversified race of Lung-fishes, whose acme of develop- 

 ment, specialization and numerical superiority occurred during 

 the Devonian. One remarkable order comprising huge armored 

 fishes passed entirely out of existence at the dawn of the Car- 

 boniferous, without leaving descendants. Such, at least, ap- 

 pears to be the most satisfactory interpretation of the group 

 now commonly known as Arthrodira. Another division, Cten- 

 odipterini, was conspicuous throughout the Palaeozoic, and at- 

 tained a higher degree of specialization along certain lines than 

 is evinced by later forms. The geological history of the Sirenoid 

 order, to which Ceratodus and its modern descendants belong, is 

 not traceable with certainty earlier than the Trias, although it 

 is not unlikely that some Palaeozoic remains, known chiefly by 

 the dentition, should properly be included here. That primitive 

 members of the Sirenoid order were in existence at least as 

 early as the Lower Devonian follows as a necessary conse- 

 quence of regarding it as ancestral to both Arthrodires and 

 Ctenodipterines. This view of their relations, however, is novel, 

 and the considerations which make for its acceptance, and com- 



* As pointed out by Haeckel, Boulenger and others, the term Dipnoi, first ap- 

 plied by Johannes Muller in 1845 for the group of Lung-fishes, is improperly so 

 used, having been previously chosen by Leuckart as a name for Amphibians. 

 There is no objection, however, to retaining the name Dipnoan as a vernacular 

 equivalent of Dipneusti, and it is here employed in that sense. 



