The Collaborative International Dictionary of English v.0.48:

Prytanis \Pryt"a*nis\, n.; pl. Prytanes. [L., fr. Gr. ?.] (Gr.
   Antiq.)
   A member of one of the ten sections into which the Athenian
   senate of five hundred was divided, and to each of which
   belonged the presidency of the senate for about one tenth of
   the year.
   [1913 Webster]