Secotium is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of fungi in the family Agaricaceae

Agaricaceae

The Agaricaceae are a family of basidiomycete fungi and includes the genus Agaricus, as well as basidiomycetes previously classified in the families Tulostomataceae, Lepiotaceae, and Lycoperdaceae. The genus contains 85 genera and 1340 species.-Genera:...

.

The members of this genus are closely related to ordinary Agaricus mushrooms, but do not open out in the usual way; this has given rise to the term "secotioid

Secotioid

Secotioid fungi are an intermediate growth form between mushroom-like hymenomycetes and closed bag-shaped gasteromycetes, where an evolutionary process of gasteromycetation has started but not run to completion...

" for such mushrooms in general. They are thought to form an evolutionary link between Agarics and Gasteroid fungi

Gasteroid fungi

The gasteroid fungi are a group of fungi in the Basidiomycota. Species were formerly placed in the obsolete class Gasteromycetes Fr. , or the equally obsolete order Gasteromycetales Rea, because they produce their spores inside their basidiocarps rather than on an outer surface...

 (whose spores are enclosed in a pouch-like structure). The widespread genus contains about 10 species that are predominantly found in warm and arid regions.

The secotioid species Agaricus deserticola used to be named Secotium texense before its strong connection to Agaricus

Agaricus

Agaricus is a large and important genus of mushrooms containing both edible and poisonous species, with possibly over 300 members worldwide...

caused it to be reclassified.