What's the difference between pygidium and pyxidium?

Pygidium

Definition:

• (n.) The caudal plate of trilobites, crustacean, and certain insects. See Illust. of Limulus and Trilobite.

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Pyxidium

Definition:

• (n.) A pod which divides circularly into an upper and lower half, of which the former acts as a kind of lid, as in the pimpernel and purslane.
• (n.) The theca of mosses.

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