Herbs, perennial, scapose, from bulbs. Bulb 1, ovoid or globose, tunicate, often extending into neck of clasping, distichous leaf bases. Leaves 2-16, deciduous or evergreen, sessile, rarely petiolate; blade narrowly to widely liguliform or oblanceolate, rarely ovate to elliptic. Scape: bracts 2-3, triangular, ovate, or lanceolate. Inflorescences umbellate, bracteate; each flower with subtending, often narrowly lanceolate bract. Flowers 1-16, usually sessile, erect or slightly diverging, large and starlike, fragrant; perianth connate basally into short or long tube, surmounted by conspicuous staminal corona; tepals extending from base of corona, free portions reflexed or ascending, often distally recurved, linear; stamens adnate basally into showy funnelform or rotate corona, margins between free portions of filaments often dentate or lacerate, portions of filaments inserted on margin of corona, erect to incurved, filiform; anthers versatile, introrse, pollen yellow, often golden, or orange; ovary inferior, globose, ovoid, oblong, or pyriform, ovules 2-10 per locule; style exserted beyond stamens, deflexed laterally, filiform; stigma capitate. Fruits capsular, green, subglobose to elongate, 3-locular, large, leathery. Seeds large, green, fleshy. x = 20, 23. The species of Hymenocallis are some of the most difficult to identify from herbarium specimens. Distinguishing field characteristics such as position, surface, texture, and color of the leaves, and three-dimensional shape and margin of the staminal corona, are undeterminable on most herbarium specimens. These characteristics, as observed in the field, are discussed in the treatment of each species, and distinguishing features are emphasized for their usefulness in identification. With the exception of dimensions for fruits, seeds, and some bulbs, all measurements presented are from pressed, dried specimens. Variations in habitat and distribution are also emphasized, and they are often used in the key to support character distinctions. The first eight Hymenocallis species treated here were classified by H. P. Traub (1962) in the Caroliniana Alliance. Species in this group primarily have 1-3 ovules per locule and deciduous, sessile, liguliform or oblanceolate leaves. They are distributed in northern Florida and throughout wetland areas of the southeastern United States. Species studied to date have a base chromosome number of x = 20. Hymenocallis rotata, H. godfreyi, H. puntagordensis, H. palmeri, and H. henryae (species 9-14) were all classified in Traub´s Henryae Alliance. They have distinctly larger ovaries than the species classified in Traub´s Caroliniana Alliance, having 4-8 ovules per locule, as well as coriaceous, suberect to erect, liguliform leaves. All species in the Henryae Alliance treated here occur in Florida. There is considerable chromosome diversity among these taxa, but the base number of many is x = 23. Hymenocallis latifolia is the only spider-lily in the flora that is classified in Traub´s Caribaea Alliance. Species in this alliance are characterized by sessile, liguliform, evergreen leaves. Many have 2-3 ovules per locule and a base number of x = 23. D. S. Correll and M. C. Johnston (1970) and D. S. Correll and H. B. Correll (1972) recognized three species of spider-lilies in Texas, and L. H. Shinners (1951) recognized two. We recommend a thorough investigation of Hymenocallis in Texas and adjacent states to achieve a clearer understanding of the southwestern spider-lily species. Like many other amaryllids, Hymenocallis species contain various alkaloids. It is not recommended that plant parts be eaten or even touched by allergic individuals (J. A. Bauml 1979). The precise localities of the bulb collections of Hymenocallis collected by Mary G. Henry from which Traub described numerous new species of Hymenocallis were determined from the field diaries of Mary G. Henry, courtesy of Josephine de N. Henry, President Emerita of the Henry Foundation for Botanical Research. This treatment would not have been possible without this critical information.