RELATIONSHIPS OF LUNG-FISHES 



127 



siren" of dealers, often of museums. It is the best known 

 of Dipnoans, on account, partly, of the ease with which it 

 may be transported alive. In the hardened mud cocoons 

 with which it encases itself during the dry season, it is 

 readily dug out of the stream bed and packed for exporta- 

 tion. When placed in tepid water, the cocoon dissolves 

 and the fish shortly revives. 



Relationships 



A review of our knowledge of Dipnoans gives but little 

 satisfactory suggestion as to their relations as a group. 

 They must certainly be looked upon as an advancing 

 phylum from which the amphibia may early have diverged. 

 Their many amphibian characters have been lately em- 

 phasized by W. N. Parker. On the other hand, the 

 evidences of the kinship of Dipnoans to the other types 

 of fishes can only be interpreted as the common con- 

 vergence of the ancient phyla toward the structures of the 

 ancestral form of fish. Thus we find that the types of 

 Devonian lung-fishes can only be distinguished from 

 those of the contemporary Teleostomes by the pattern 

 of arrangement of the plates of the head roof,* a condition 

 which has led Smith Woodward to believe that these 

 groups had already diverged before the appearance of 

 dermal bones. 



Lung-fishes have unquestionably many structures which 

 ma}/' have been derived from the more generalized condi- 

 tions of the sharks ; and as a group they may not unrea- 

 sonably be looked upon as descended from the primitive 

 elasmobranchian stem. Their ties of kinship to the sharks 



* The present writer regards this distinction as somewhat provisional; 

 median head plates are nominally characteristic of Dipnoans (Fig. 124), but, 

 as in the sturgeons and siluroids, they are also well known among Teleostomes. 

 Protopterus has, moreover, a symmetrical arrangement of the head plates. 



