The Isoetopsida is a class of the Lycopodiophyta. All living species belong to the genus Selaginella in the Selaginellales or to Isoetes in the order Isoetales. In the past, members of this group have sometimes been placed in the class Isoetopsida, sometimes in the Selaginellopsida or Lycopodiopsida. There are about 700 species of Selaginella and 140-150 species of Isoetes, with a cosmopolitan distribution but often scarce to rare. Some botanists split Isoetes, separating two South American species into the genus Stylites.
Some workers prefer the name Selaginellopsida A.B. Frank 1877, which has priority over the name Isoetopsida; the latter was not published until 1885. However, priority does not apply above the rank of family. Recent articles favor the name Isoetopsida, since Selaginellopsida sometimes is ambiguously used. It may have the same membership as the Isoetopsida described here, or it may include only the order Selaginellales.
The most famous group within the Isoetopsida is the "scale trees" (order Lepidodendrales), which include Lepidodendron. These massive trees flourished in marshlands of the Carboniferous. Quillworts are considered by some to be the last remnant of such fossil trees, with which they share some unusual features including the development of both wood and bark, a modified shoot system acting as roots, bipolar growth, and an upright stance.