GENERAL CHARACTERS. 317 



are shown in the skeleton represented on p. 316, and occur in both the median and 



paired fins, of which the names are also given in the same illustration. In the 



median fins the bases of these rays articulate with the interspinal bones, or, in 



clasmobranchs, with the radial cartilages. The first 



rays of the pectoral and dorsal fins may be developed 



into long spines, having the same structure as teeth, 

 internal In the internal skeleton the back- 



Skeleton, bone is divisible only into a trunk and 



caudal moiety. In the fringe -finned ganoid fishes 



the primitive notochord persists, although it may be 



partly surrounded by rudimental arches; while in the P 



sharks and higher bony fishes the column is divided I 



into segments, forming vertebrae with doubly-cupped 



bodies. In sharks and rays the arches and bodies of ^ 



the vertebrae remain separate, but in the other groups \ 



they are fused together ; in the tail, as shown in our j 



figure of the skeleton of the perch, there is also an \ 



inferior arch and spine to each vertebra. In the more \ 



primitive fishes the notochord is continued to the \ 



hinder extremity of the body, where it is surrounded | 



symmetrically by the rays of the caudal fin; this type, [ 



which is shown in the accompanying figure of the j 



skeleton of an extinct fringe - finned shark, being ; 



termed the fringe-tailed, or diphycercal. Whereas in 



some fishes with this type of tail the fringes on the \ 



upper and lower portions of the caudal fin are of ! 



nearly equal depth, in others the lower fringe of rays 



becomes somewhat deeper than the others, and a 



further development of this inequality results in the 



partially forked or heterocercal tail of the modern j 



sharks and sturgeons, where the end of the backbone 



is bent upwards into the longer superior lobe of the 



tail, the lower lobe of which is formed exclusively 



of rays. The lung -fishes and sharks have never 



advanced beyond one or other of these types ; but - 



the bony fishes and ganoids, which started with the 



primitive fringed lobate type, by a gradual shortening 



of the central part of the tail-fin, accompanied by an 



increasing development of the rays on its lower side, 



have evolved the completely forked or homocercal tail 



of the perch, in which, as shown in the figure, the 



backbone stops short of the fin-rays, and ends in an 



expanded, unsymmetrical extremity, from which these rays are given off in a 



fan-like manner, so as to produce an appearance of perfect symmetry in the whole 



structure. 



Turning to the limbs, or paired fins, we find that while in the existing 



