3 l8 
FISHES. 
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elasmobranchs there are no membrane-bones (as the elements of the skeleton not 
formed from primitive cartilage are termed), in the higher bony fishes the pectoral 
girdle, as shown in the figure on p. 316, comprises a scapula and a coracoid, 
flanked by a series of membrane-bones, known as the 'post-temporal, supra¬ 
clavicular, clavicular, and postclavicular. The pelvis 
is generally absent, and is never highly developed. In 
all cases the basal and radial bones of the pectoral fins 
articulate directly with the pectoral girdle, so that there 
are no segments corresponding to the arm and fore-arm 
of the higher Vertebrates. In the paired fins the struc¬ 
ture is very similar to that of the tail; and a similar 
transition from a fringed to a fan-like type may be 
traced as we pass from the primitive to the specialised 
forms. For instance, in the figure of the perch’s 
skeleton on p. 316, we may notice that the paired fins 
are formed of a number of hard rays spreading out in 
a fan-like manner from a single point of origin; and 
the same general type obtains in the existing sharks and 
rays. In certain extinct sharks, like the one of which 
the skeleton is shown on p. 317, as well as in the lung- 
fishes and the fringe-finned ganoids, the pectoral fins 
have a long central lobe running for some distance up 
the middle, and completely covered with scales (where 
these are developed), while the rays of these fins form 
a kind of fringe radiating on all sides from the central 
lobe. The skeleton of such a fin, which is known as 
an archipterygium, consists of a long cartilaginous axis, 
composed of a number of joints, gradually decreasing in 
size from the base to the extremity, as shown in the 
figure on p. 319. From one or both sides of such 
joints there are given off a number of oblique smaller 
jointed rods, terminating in the fine rays forming the free 
edges of the fins. How different is the structure of this 
fin from that of the higher bony fishes will be apparent 
by comparing the accompanying figure with that of the 
skeleton of the perch on p. 316. In the lung-fishes this 
primitive type of fin has persisted to the present day; in 
the sharks it has now totally disappeared; while among 
the bony fishes and ganoids, in the latter of which it was 
the universal type at the period of the Old Red Sandstone, 
it now only remains in a modified form in the bichir of 
the Nile, having been developed in the modern bony fishes into the fan-type. It 
may be mentioned that the latter modification of fin is obviously the one best 
adapted for quick-swimming fishes, the fringe-finned type partaking more of the 
nature of clumsy paddles, and being adapted for slowly-moving forms like the 
lung-fishes, which pass most of their time among the mud at the bottom of rivers. 
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