SPINE-FINNED GROUP. 
n ** a 
03 4 
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 (J Rat. 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 
Fishes. 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 tlie 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. 
