1886.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



77 



[for it i.s found, in the first place, that, as a rule, the median fins, for 

 [ example, l)ejj;in their development as uninterrupted folds. Only one 

 type, the Dipnoi, has retained this pre-eminently embryonic character, 

 all other fish-like brauchiferous Chordata (not considering the Lepto- 

 cardians and ^Farsipobraiichs) very soon show a tendency to depart 

 from such a primitive condition. In consequence there is atrophy of 

 |i certain portions of the fold, while others hypertrophy, and the in- 

 cluded actinotrichia become involved, and new features arise from prim- 

 itive ones, some being actually superimposed upon older ones. 

 Il It is difficult to arrange the groups of fishes in a satisfactory way by 

 ; the help of any one character or group of characters, and it is especially 

 difficult to indicate by the help of an ideal tree what seem to be their 

 I actual genetic relationships. The best way to indicate changes in the 

 I grade of development would be to consider the most embryonic form 

 J nearest to an ideal type from which there has been divergence and 

 i specialization in various directions, thus : 



Primitive type. 



Dipnoi. 



n.iiocfpiiMi 



1 * Croseopterygii. 



t* Physoetomous 



Teleostei. 



f * CycIoganoideL 



tPliysclistona 



Teleostei. 



The groups indicated by an asterisk are physostomous, and in the 

 diagram the length of the diverging lines is intended to show the de- 



