Variation in the rut-muraria complex needs further research (FNA 2, W). North American plants are tetraploid (2n = 144) and generally known as var. cryptolepis (Fern.) Wherry. They are largely restricted to calcareous cliffs in eastern regions. Plants with more acute leaf segments may be named A. ruta-muraria var. lanceolum Christ (= A. cryptolepis Fern. var. ohionis Fern.), but their taxonomic status has been doubted (Cranfill 1980; FNA 2). Such plants are known from BULL (DHL, NCU), CART (KY), JEFF (DHL), POWE (D. Dourson, pers. comm.) and elsewhere. Atypical plants from MCRE (MM for WKY), on the ridge at the mouth of Little South Fork, have ususual filiform pinnules, and may deserve taxonomic attention (M). Note that hybrids of cryptolepis with rhizophyllum or other species are "exceedingly rare" in North America (FNA 2).