ACROSTICUM, RUSTYBACK, WALL-RUB, or FORK-FERN, in botany, a genus of the cryptogamia filices. The fructifications are accumulated on the whole inferior surface of the frond, so that they everywhere cover it. There are upwards of 30 species; but only three of them (according to others, two) are natives of Britain, viz. the septentrionale, or horned fern, which grows on walls or cliffs of rocks; the ibenise, or hairy fern, growing in cliffs of rocks; and the thelypteris, or marsh-fern, in turfy bogs.