Calamites is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails (genus Equisetum) are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to a height of 10 meters (31 feet). They were persistent minor component of coal swamps of the Carboniferous period.

A Mississippian to Permian fossil plant. A scouring rush that grew to be 40-feet high, it had a ribbed, jointed trunk with leaf whorls at the joints.