Autor: Buxbaum & Oehme

“Turnip Cactus”, referring to the shape of the root in this genus, a long taproot.
A genus of small plants, globose to elongated, constricted towards the base, usually solitary, with the apex woolly. Large taproot, often exceeding the volume of the plant above the soil, often hoisted on a very thin neck (R. subterraneus). Ribs absent, divided into tubercles. Areoles situated at the tip of tubercles, woolly, white, having dense, straight and sharp spines.
Flowers diurnal, self-sterile, appearing at the apex of stems, funnel-shaped, white to pink, pollinated by insects, mainly bees and flies. Fruits berry-like. Seeds striate, with hllum and micropyle disjoint.
With its napiform root, the genus Rapicactus can live in extreme ecological niches, dry areas where little or no other cacti grow, mimetic, almost invisible, on soils or walls of calcareous rocks, sometimes on acid soils (R. beguinii), from approximately 1000 m up to 3200 m in altitude.
Mexico (Coahuila, Nuevo Leon, San Luis Potosi, Tamaulipas, Zacatecas).

Currently 6 recognised species + one possible subspecies:
– Rapicactus beguinii* (N. P. Taylor) Luthy 2003
– Rapicactus beguinii subsp. hintoniorum (Hofer) Luthy 2003
– Rapicactus booleanus* (Hinton) Donati 2003
– Rapicactus mandragora* (Fric ex. Berger) Buxb & Oehme 1943
– Rapicactus pailanus (Halda & Panarotto) Donati 2003
– Rapicactus subterraneus* (Backbg.) Buxb & Oehme 1942
– Rapicactus zaragozae* (Glass & Forster) Donati 2003

References: "TAXONOMY of the CACTACEAE" -  ISBN 978-84-617-3692-8 (Vol. 2)