H. nigricans sensu lato occurs mostly in northern Mexico and south-central states, from Tex. and La. to Neb. and Iowa (K under Stenaria). East of the Mississippi Rv. there are several disjunct clusters of records, which appear to be somewhat distinct in most cases (W), but deeper revision is needed. Terrell (2001) did not distinguish the taxon mapped here, which has no established type coll. But Rafinesque referred to it four times (Merrill 1949) and there is a potential type at NY (acc. no. 00131898): [collector not stated] "Houstonia rupestris Raf. ... ad rupes. fluvi. Kentucky" [cliffs bordering Kentucky Rv.]. It also appears to be the unnamed Houstonia of Short & Peter (1835): "only among the rich debris of the limestone cliffs bordering the Kentucky river, where rooting deeply in the fissures of the rock, it forms dense clumps of considerable magnitude, with numerous semi-prostrate stems a foot long." Based on a review of colls. at GH, MO, NY and elsewhere, this variant of nigricans appears largely restricted to limestone cliff-tops along the Kentucky River Palisades, the Green Rv. in Ky., the Cumberland Rv. in Ky. and Tenn., and perhaps the Sequatchie Valley in Tenn. (KY, APSC, TENN). The largest leaves in each plant (generally at mid-stem) are ca. 3-6 mm wide, and there is less reduction into the inflorescence; axillary fascicles of narrow leaves are lacking. In typical nigricans, the largest leaves are usually 1-2.5 (3) mm wide; leaves are much reduced into the inflorescence; and axillary fascicles are usually present at mid-stem. The broad-leaved variant also appears to have a more decumbent, spreading habit and less congested inflorescences, with about 5-10 pure white flowers per square cm in pressed specimens; typical nigricans has about 10-20 pale bluish flowers per square cm. More analysis is needed to determine the degree of distinction. With some biogeographic parallels, cryptic variants may also exist among associated xerophytic calciphiles in Sedum, Sabulina / Mononeuria, Cerastium, Rudbeckia, Allium and Sporobolus.