Psilotopsida is a class of fern-like plants.1] It should not be confused with the obsolete class Psilophytopsida. As circumscribed by Smith et al. (2006) Psilotopsida contains two families, Psilotaceae and Ophioglossaceae, placed in orders Psilotales and Ophioglossales, respectively. The affinities of these two groups have long been unclear and a close relationship between them has only recently been confirmed through molecular systematic studies. Psilotopsida is the sister-group to all other ferns (including Marattiaceae and Equisetaceae).
Pryer, Kathleen M., Eric Schuettpelz, Paul G. Wolf, Harald Schneider, Alan R. Smith and Raymond Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91:1582-1598 (online abstract here).
Ophioglossales (lit. 'snake-tongue-plant') are a small group of pteridophyte plants. Traditionally they are included in the division Pteridophyta, the ferns, originally as a family and later as the order Ophioglossales. In some classifications this group is placed in a separate division, the Ophioglossophyta, but recent molecular systematic studies have shown the Ophioglossales to be closely related to the Psilotales. The most recent classification by Smith et al. (2006) places these two orders together in class Psilotopsida. [more]
Psilotaceae is a family of fern-like plants (in order Psilotales) consisting of two genera, Psilotum and Tmesipteris. The two genera are very different and in the past Tmesipteris has been placed in its own family, Tmesipteridaceae, but most classifications continue to treat it in Psilotaceae. The relationships of Psilotaceae have been unclear, in part because the plants lack roots or true leaves, but recent molecular systematic studies suggest a relationship to the fern family Ophioglossaceae. [more]
At least 76 species and subspecies belong to the Order Psilotales.
More info about the Order Psilotales may be found here.