Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Stems cylindric to globose, solitary or forming clumps, not jointed, covered with cylindric to conic tubercles, each of these with a terminal areole bearing radial and usually also central spines but lacking glochids; fls arising individually on new growth of the season at the base of an adaxial groove descending from the areole; hypanthium funnelform- prolonged above the ovary; fr thin-walled, fleshy; seeds small, dark, wingless. 30+, N. Amer.