MARCHANTIA, in botany, a genus of the cryptogamia algae class. The calix of the male is peltated, and covered below with monopetalous corollæ; the calix of the female is sessile, bell-shaped, and contains many seeds. There are eight species, four of them natives of Britain, viz. the polymorpha, or common marchantia; the cruciata, or cross-headed marchantia; the hemisphaerica, or marsh marchantia; and the conica, or wart marchantia.