Source : Webster's Revised Unabridged Dictionary (1913)

Pentamerous \Pen*tam"er*ous\, a. [Penta- + Gr. ? part.]
   1. (Biol.) Divided into, or consisting of, five parts; also,
      arranged in sets, with five parts in each set, as a flower
      with five sepals, five petals, five, or twice five,
      stamens, and five pistils.

   2. (Zo["o]l.) Belonging to the Pentamera.

Source : WordNet®

pentamerous
     adj : divided into five parts; specifically, having each floral
           whorl consist of five (or a multiple of five) members;
           "pentamerous flowers"