334 



SPINE- FINNED GROUP. 



Fan-Finned This group — Actinopterygii — includes all the bony fishes of the 



Group. present day, as well as the sturgeons, and is characterised by the 

 fan-like structure of the paired 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 eight suborders of this order, given 

 in the table on p. 333, form one great division characterised by the number of 

 dermal rays in the dorsal and anal fins being equal to that of the supporting 



PIKE-PERCH AND COMMON PERCH [\ Hat. size). 



bony elements, and by the tail being never heterocercal, 1 but usually either of the 

 abbreviate-heterocercal or homocercal type, although occasionally diphycercal. 

 Spine-Finned In the classification proposed by Professor Cope the first four 



Pishes. suborders of the fan-finned group given in the foregoing table are 

 regarded as a single group, under the title of Physoclysti, and, in common with 

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

 without a spiral valve, and the skeleton fully ossified. From the Physostomi, the 



1 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 vertebra; are continued without abortion 

 along the middle line of the symmetrical tail-fin. 



