The Collaborative International Dictionary of English v.0.48:

Peristoma \Pe*ris"to*ma\ (p[-e]*r[i^]s"t[-o]*m[.a]), n.; pl.
   Peristomata (p[e^]r`[i^]*st[o^]m"[.a]*t[.a]). [NL.]
   Same as Peristome.
   [1913 Webster]