The Collaborative International Dictionary of English v.0.48:

Pygidium \Py*gid"i*um\, n.; pl. Pygidia. [NL., fr. Gr. ?, dim.
   of ? the rump.] (Zool.)
   The caudal plate of trilobites, crustacean, and certain
   insects. See Illust. of Limulus and Trilobite.
   [1913 Webster] Pygmy