The same thing to a less degree takes place in reptiles; the ankle-bones do not indeed anchylose with the shin-bone and foot respectively, but they nevertheless unite with those parts so firmly that motion takes place between the bones of the ankle and not between the whole ankle and the leg. From Wordnik.com. [The Common Frog] Reference
In all birds, on the contrary, not only is there no motion between the ankle-bones (as a whole) and the shin-bone, but the two rows of ankle-bones actually anchylose respectively with adjacent parts -- the row nearer the leg coming to form one with the shin-bone; the second row coming to form one with the bones of the foot. From Wordnik.com. [The Common Frog] Reference
It is fish-like because the terminal piece, as it is called, or "coccyx" (unlike the coccyx in man or in birds) is not composed of rudimentary vertebræ which subsequently blend and anchylose together, but is formed by the ossification continuously of the membrane investing (or sheath of) the hindermost part of that primitive continuous rod, or notochord, 16 which, as has been said, precedes, in all vertebrate animals, the development of the backbone, making its appearance beneath the primitive groove. From Wordnik.com. [The Common Frog] Reference
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