Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Fls perfect or unisexual, in small lateral clusters on twigs of the current year; stamens 4, scarcely exsert; hypanthium tubular-turbinate above the globose, investing base, lobed not more than ca half-length, deciduous from the top of the developing fr; fr drupe-like, the persistent hypanthium-base rather dry and mealy, with a hard, bony, striate-fluted inner layer; lvs alternate. 45, Eurasia, N. Am.