The Collaborative International Dictionary of English v.0.48:

Perigonium \Per`i*go"ni*um\, n.; pl. Perigonia. [NL.]
   Same as Perigone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perigonium
    n 1: collective term for the outer parts of a flower consisting
         of the calyx and corolla and enclosing the stamens and
         pistils [syn: perianth, chlamys, floral envelope,
         perigone, perigonium]