What is "gymnocarpium"

Gymnocarpium is a small genus of ferns, called oak ferns. It was once placed with various other groups, including the dryopteroid ferns and the athyrioid ferns. Cladistic analysis has demonstrated that Gymnocarpium and Cystopteris form a natural but relatively primitive clade that is basal to the asplenoid, thelypterioid, and athyrioid ferns comprehensively.

Gymnocarpium sori are small, round and naked. These ferns generally have a slender, creeping rhizome under the surface of the ground, and fairly thin-textured fronds.