Hypertext Webster Gateway: "pygidium"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Pygidium \Py*gid"i*um\, n.; pl. {Pygidia}. [NL., fr. Gr. ?, dim.
of ? the rump.] (Zo["o]l.)
The caudal plate of trilobites, crustacean, and certain
insects. See Illust. of {Limulus} and {Trilobite}.
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