63 



The main line then passed upwards towards the higher 

 Fishes. This ancestral series above the common origin of 

 the Elasmobranchii and Holocephali, is the Proto-Ganoidei 

 of Balfour, * called Pneumatocoela by Bridge f because the 

 air-bladder probably first appeared as a distinct organ in this 

 group. From it the primitive Ganoids and Teleosteans 

 diverged together. 



The Teleostei are the most highly differentiated group of 

 Fishes, and have been derived from the ancestral Ganoidei. 

 The endo skeleton which was cartilaginous in the Elasmo- 

 branchii and Proto-Ganoidei, becomes more or less com- 

 pletely ossified in the higher Fishes, and especially in the 

 Teleostei. In many respects the Ganoidei more closely 

 resemble the common ancestry. They have separated into 

 two groups, of which one retains in a less modified condition 

 the primitive characteristics, while the other resembles the 

 Teleosteans in the differentiations which have been effected. 

 The Teleostei have been evolved into a number of distinct 

 groups, some of which have diverged considerably from the 

 common ancestor. 



The Dipnoi must be regarded as representing an offshoot 

 from the Proto-Ganoidei at the point where the common 

 ancestors of the Ganoids and Teleosteans diverged from the 

 line leading upwards to the higher Yertebrata. In some 

 respects the Dipnoi exhibit features seen in the less modified 

 Ganoidei and the Elasmobranchii, and in virtue of such 

 characters they are persistent Proto- Ganoids; but on the 

 other hand, the Dipnoi show in some points that they have 

 undergone evolution in the direction of the line leading 

 towards the primitive Amphibians. The most important 

 character in which they exhibit an advance upon other 

 groups of Fishes is the development of the air-bladder, which 



* Comp. EmhryoL, v. ii, p. 272. 

 t Fhil. Trans., 1878, Part ii. 



