CLASS PISCES. 



Sub-Classes. 



1. Elasmobranchii. 



2. Holocephali. 



3. Dipnoi. 



4. Teleostomi. 



Among the fishes existing at the present day there may be 

 observed two distinct plans of cranial structure, between which 

 DO definitely intermediate conditions can be recognized. In 

 Chinuera, Protopterus, Ceratodus, and .their allies, the upper 

 segment of the mandibular arch is directly fused with the 

 chondrocranium, while the corresponding segment of the hyoid 

 arch is atrophied or absent ; in the Elasmobranchs and the so- 

 called " Ganoidei " and " Teleostei," the same elements are 

 loosely articulated with the chondrocranium, the upper segment 

 of the hyoid arch forming a moveable susgensorium. The first 

 condition is now commonly known as the autostylic, and the 

 second as the hyostylic ; the former attaining its highest phase 

 of development as fishes pass into batrachians, and the latter as 

 the Palaeozoic forerunners of the typical modern fishes reach 

 w. 2 



