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Updated: May 5, 2025
The many-toed fin had been used almost solely for motion in the water; it had now also to support the body in creeping on the solid ground. This led to a modification both of the skeleton and the muscles of the limbs. The number of the fin-radii was gradually reduced, and sank finally to five. But these five remaining radii became much stronger. The soft cartilaginous radii became bony rods.
In the first year the two osseous halves of the arches unite; but it is much later in the second to the eighth year that they connect with the osseous vertebral bodies. A, B, cartilaginous series of the fin-stem. rr cartilaginous fin-radii. R, R, radii of the lateral fin-border, mt metapterygium, ms mesopterygium, p propterygium. Skeleton of the breast-fin of a young Selachius.
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