J. K. Thacher — Ventral Fins of Ganoids. 



241 



The Stapedifera together with the Dipnoi constitute a genetic 

 group. They liave posterior nares, or choanae, opening within the 

 mouth. We may then call them Choanata. 



With a less degree of certainty, I should regard the Elasmobranchii, 

 Teleostei, and Dipnoi, as genetic groups : as also a group formed by 

 withdrawing the Chondrostei from the Gephyrrhina. These relations 

 are exhibited in the following table. 



Protogna thostomi . 



It will be noticed that in two places in this table the division is 

 into more than two lines, into three in one case and into five in the 

 other. I do not intend to indicate that any such multiplicate fission 

 really took place, but merely that it is not yet evi- 

 dent exactly how these sub-gi'oups are related to 

 one another. The relation of the genetic groups 

 A, B, and C may be that exhibited by making C's 

 line come in at X', X" or X'". But while it is prac- 

 tically impossible that X shoiild coincide with O, it 

 is very easy for X to get so near O as to be indis- 

 tinguishable from it, and this Avill depend principally 

 on the amount of change that has taken place between X and O, and 

 on the completeness of our knowledge of the results of that change as 

 now exhibited in living or fossil forms. To represent the lines as 

 Trans. Conn. Acad., Vol. IV. 31 Dec, 1877. 



