GENERAL CHARACTERS. 
3*7 
are shown in the skeleton represented on p., 316, and occur in both the median and 
paired tins, of which the names are also given in the same illustration. In the 
median tins 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- 
Skeieton. 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 | 
sharks and higher bony fishes the column is divided g 
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 h 
they are fused together; in the tail, as shown in our 3 
figure of the skeleton of the perch, there is also an g 
inferior arch and spine to each vertebra. In the more | 
primitive fishes the notochord is continued to the 2 
hinder extremity of the body, where it is surrounded g 
symmetrically by the rays of the caudal fin; this type, gj 
which is shown in the accompanying figure of the g 
skeleton of an extinct fringe - finned shark, being w 
termed the fringe-tailed, or diphycercal. Whereas in § 
some fishes with this type of tail the fringes on the I 
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 | 
sharks and sturgeons, where the end of the backbone 
is bent upwards into the longer superior lobe of the 3 
tail, the lower lobe of which is formed exclusively * 
of rays. The lung - fishes and sharks have never g 
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 homocerccd 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 
