The Collaborative International Dictionary of English v.0.48:

Monosepalous \Mon`o*sep"al*ous\, a. [Mono- + sepal: cf. F.
   monos['e]pale.] (Bot.)
   Having only one sepal, or the calyx in one piece or composed
   of the sepals united into one piece; gamosepalous.
   [1913 Webster]

   Note: The most recent writers restrict this term to flowers
         having a solarity sepal, and use gamosepalous for a
         calyx formed by several sepals combined into one piece.
         Cf. Monopetalous.
         [1913 Webster]