Annuals, biennials, or, rarely, perennials; not scapose; glabrous or pubescent. Stems (simple or few to several from base), erect [ascending, decumbent], branched basally and/or distally, (glabrous or pubescent). Leaves basal and cauline; petiolate or sessile; basal (soon caducous), rosulate or not, petiolate, blade margins entire or dentate; cauline sessile, blade (base auriculate, sagittate, or amplexicaul), margins entire or dentate. Racemes (corymbose, dense or lax), considerably [slightly] elongated in fruit. Fruiting pedicels erect to ascending or divaricate, slender. Flowers: sepals (equal), usually erect, sometimes ascending or spreading, rarely reflexed, oblong, lateral pair not saccate basally; petals (erect basally), usually white, lavender, or purple, rarely yellow, oblanceolate, oblong, spatulate, or obovate, (margins rarely crisped), claws differentiated or not from blade, (apex rounded); stamens tetradynamous, (exserted); filaments distinct; anthers usually linear, sometimes ovate or oblong (apically coiled); nectar glands confluent, subtending bases of stamens. Fruits stipitate, linear, torulose or smooth, terete; valves each with prominent midvein, usually glabrous, rarely pilose; replum rounded; septum complete; ovules 20-114 per ovary; style distinct, (cylindrical or subclavate to clavate); stigma capitate, usually strongly 2-lobed, rarely entire, (lobes opposite replum). Seeds uniseriate, plump, not winged, oblong; seed coat (minutely reticulate), not mucilaginous when wetted; cotyledons incumbent or obliquely so. x = 10, 11.
As recognized by R. C. Rollins (1982b, 1993), Thelypodiopsis was artificially delimited, and no single characteristic or combination of characteristics reliably distinguished it from related genera. Rollins´s circumscription was so broad that it included species of Dryopetalon and Romanschulzia O. E. Schulz.
Of the nine species of Thelypodiopsis that occur in the flora area, three (T. purpusii, T. shinnersii, and T. vaseyi) are quite anomalous and, together with the Mexican T. versicolor (Brandegee) Rollins (Coahuila, San Luis Potosí), eventually may be excluded from the genus. Unlike the other six species, these three have entire or obscurely (versus prominently) 2-lobed stigmas, oblong or ovate anthers 0.5-1 mm (versus linear and 2.5-4 mm), and petal claws obscurely (versus strongly) differentiated from blades. They are here retained in the genus only tentatively and currently are being subjected to detailed molecular and morphological studies, along with Mexican, Central American, and South American taxa.