Isoëtes, also written Isoetes and commonly known as the quillworts, is a genus of plants in the class Isoetopsida

Isoetopsida

The Isoetopsida is a class of the Lycopodiophyta. All living plants 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...

 and order Isoetales

Isoetales

Isoetales, also written Isoëtales, is an order of plants in the class Isoetopsida. There are about 140-150 living species, all classified in the genus Isoëtes , with a cosmopolitan distribution but often scarce to rare. Living species are mostly aquatic or semi-aquatic in clear ponds and...

. They are considered "fern allies

Fern ally

Fern allies are a diverse group of seedless vascular plants that are not true ferns. Like ferns, a fern ally disperses by shedding spores to initiate an alternation of generations.-Classification:...

". There are about 140-150 species, with a cosmopolitan distribution

Cosmopolitan distribution

In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...

 but often scarce to rare. Some botanists split the genus, separating two South American species into the genus Stylites.
Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several (e.g.