Limb bones within unrelated animals that have the same basic structures are considered

If the animals are not closely related, but the structures are meant to serve the same function (structure determines function!) then the structures are analogous. The difference between analogous and homologous is that in analogous structures, the creatures are not closely related but the structure serves the same purpose (like bat wings/bird wings) and homologous structures are similar structures directly as a result of ancestry.

Gay-lussac's law i think.