WordNet (r) 3.0 (2006):

Psilopsida
    n 1: whisk ferns; comprising the family Psilotaceae or
         Psilotatae: vascular plants with no roots, partial if any
         leaf differentiation, and rudimentary spore sacs [syn:
         Psilopsida, class Psilopsida, Psilotatae, class
         Psilotatae]