colouration is closer to that of N. spectabilis. Nepenthes rigidifolia differs from N. bongso, N. ovata and related species in having mostly ovoid upper pitchers (compared to infundibular in the others), distinctly thick and coriaceous laminae, and a narrower, cylindrical peristome with very short teeth. In addition, the lower pitchers of N. bongso are considerably larger than those of N. rigidifolia. While recognising N. rigidifolia as a valid species in his Carnivorous Plant Database, taxonomist Jan Schlauer suggests that it may be conspecific with N. densiflora. Terrestrial pitchers of N. rigidifolia may superficially resemble those of N. spectabilis, but it can be distinguished from this species on the basis of its trap colouration, upper pitcher shape (largely ovate in N. rigidifolia versus predominantly cylindrical in N. spectabilis), thinner leaves, and branched spur. Natural hybrids Only one natural hybrid involving N. rigidifolia (with N. spectabilis) is known. This cross, which is only known from a single open rocky outcrop, may now outnumber N. rigidifolia at the type locality (as observed by Stewart McPherson in 2007). The hybrid differs from N. rigidifolia in having narrower pitchers with a strongly infundibular base and distinct hip around the middle. On the other hand, the pitchers of this hybrid are broader than those of N. spectabilis and have an expanded peristome, as well as a markedly shorter spur. Charles Clarke wrote of this cross: "Unlike N. ovata × N. spectabilis (from Gunung Pangulubao), this hybrid seems to combine the more attractive characteristics of both parent species, producing large, beautifully coloured pitchers". The richly coloured lower pitchers of N. rigidifolia × N. spectabilis may superficially resemble those of N. macfarlanei. However, since that species is confined to Peninsular Malaysia, the two taxa are not easily confused. Notes a.The Latin diagnosis for N. rigidifolia reads: Folia sessilia crassa rigide coriacea laminis ovatis ad spathulato-oblongis 17.8–20.2 cm longis 5.6–7.8 cm latis venis longitudinalibus 3 vel 4, cirrho subapicali 0.3–0.9 cm infra apicem inserto, ascidium superius ovoidissimum 20.7–21.1 cm altum 7.2–9.4 cm latum, peristome expanso 2.2–4.4 cm lato, ambobus lateris 4-lobatis antice incisura, collifero, operculo ovato, calcare trifido, intus pubescenti, inflorescentiae pedunculus rami pedicelli bracteoli tepali filamenta pubescentes, pedunculus c. 4.2 cm longus, ramis 0.4–0.5 cm longis, pedicelli 0.5–0.6 cm longis bifloris. ─ Typus: Sumatra Utara, Kab. Karo, around Sidikalang areas, 1000–1500 m, Dec 11, 2003, Nepenthes Team (Hernawati, P. Akhriadi & I. Petra), NP 354 (‘ANDA’–Holo, BO–Iso). References