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THE BONY FISHES AND GANOIDS 



Fan-Finned 

 Group 



fins, in which the proper internal skeleton is abbreviated to make way 

 for the greatly developed dermal fin rays; the caudal fin being of very 

 variable structure. In the branchiostegal membrane, occupying the 

 space between the two branches of the lower jaw, there is always a paired series of 

 transversely elongated rays. The first seven suborders of this order, given in the 

 table on p. 2701 , form one great division characterized by the number of dermal rays 

 in the dorsal and anal fins being equal to that of the supporting bony elements, and 





PIKE PERCH AND COMMON PERCH. 



(One-fourth natural size.) 



by the tail being never heterocercal,* but usually either of the abbreviate-heterocer- 

 cal or homocercal type, although occasionally diphy cereal. 



In the classification proposed by Professor Cope the first four sub- 

 Fishes or ders of the fan-finned group given in the foregoing table are 

 regarded as a single group under the title of Physoclysti, and in com- 

 mon with the tube-bladdered fishes, have the fibres of the optic nerves interlacing, 



* In the heterocercal type the upper lobe of the tail is the longer, and the vertebral column is continued up into 

 it; in the abbreviate heterocercal the tail is symmetrical, and the vertebral column complete but bent up into its 

 upper half; in the homocercal type the tail is also symmetrical, but the vertebrae stop short at its base, where the 

 latter ones are aborted into a mass; in the diphycercal form the vertebrae are continued without abortion along the- 

 middle line of the symmetrical tail fin. 



