58 A HISTORY OF FISHES 



instead of horny fibres the outer part is supported by bony 

 fin-rays. The conversion of the fibres into bony supports 

 obviously lends additional strength to the fin, and in order to 

 retain the necessary flexibility, these are segmented into a 

 number of sections. In all the higher Bony Fishes the lobe at 

 the base of the fin disappears, the radials are reduced to mere 

 nodules of bone or cartilage sunk within the adjacent muscles 

 of the body, and the sole support of the fin is provided by the 

 bony fin-rays. The basals, which may be joined to the reduced 

 radials, persist as a series of fine rod-like structures alternating 

 with the spines of the backbone, and are, therefore, known as 

 interspinous bones (Fig. 24c). The bony fin-rays being de- 

 veloped in the skin on either side of the fin are necessarily of a 

 double character, and, as will be shown later on, are of two 

 kinds, spines and soft-rays. It may be noted here that in the 

 Lung-fishes {Dipneusti) and Sturgeons (Acipenseridae) , just as in 

 the Selachians, the fin-rays are much more numerous than the 

 supporting radials ; in all the higher Bony Fishes the former are 

 reduced in number and equal to the radials. 



The paired fins, owing to the need for free movement in 

 various directions, have naturally been more modified than 

 the median fins, and in living fishes little trace remains of the 

 primitive parallel arrangement of the supporting radials and 

 basals. It would be out of place to follow all the changes in 

 detail in a work of this nature, and reference must be made to 

 the zoological text-books for further information on this matter 

 {cf. p. 435). It must suffice to point out that the basals and 

 radials have been variously crowded together and fused, to 

 provide not only a strong supporting axis for the remainder of 

 the fin, but also fin-girdles designed to connect the fins with 

 the body (Fig. 65). As might be expected from their more 

 important position, evolution has proceeded considerably 

 further in the pectoral fins than in the pelvics, and its course 

 has been mainly in the direction of a progressive shortening of the 

 base, in order to permit of free movement in diflferent planes. In 

 the Selachians this shortening has been effected by an outward 

 rotation of the main basal supporting cartilage (Fig. 2b), and in 

 the Bony Fishes by a simple crowding together and reduction of 

 the supporting elements. Some of the more interesting types of 

 pectoral fin structure are shown in the accompanying diagrams 

 (Fig. 25). It will be observed that in the Australian Lung-fish 

 (Epiceratodus) there is a definite central axis to the fin formed of 

 basals, with a series of projecting radials on either side. 



