zygnemidhttp://www.oed.com/dictionary/zygnemid_n%3Ftab%3Dmeaning_and_usezygnemid, n.
First published 1921; not fully revised

zygnemidnoun

Botany.
  1. 1887–
    A member of the taxonomic order Zygnemaceae of filamentous freshwater algae.
    1. 1887
      The Conjugatæ, including zygnemids and desmids.
      Athenæum 12 March 357/1

Originally published as part of the entry for zygnemaceous, adj.

zygnemaceous, adj. was first published in 1921; not fully revised.