Summary. A new species of Ariocarpus, A. bravoanus, is described. This species, which is known only from a single locality in San Luis Potosí, Mexico, has an array of morphological features reflecting an intermediate position between the two subgenera recognized within the genus: subgen. Roseocactus and subgen. Ariocarpus; however, A. agavoides appears to be its closest relative.

Resumen. Se describe una nueva especie de Ariocarpus, A. bravoanus. Esta especie, la cual se conoce de una sola localidad de San Luis Potosí, México, tiene una serie de caracteres morfológicos que la ubican en una posición intermedia entre los dos subgéneros reconocidos dentro del género: Subgen. Roseocactus y Subgen. Ariocarpus; sin embargo, parece estar más íntimamente relacionada con A. agavoides.

Zusammenfassung. Eine neue Art der Gattung Ariocarpus, A. bravoanus, wird beschrieben. Diese Art, die nur von einer einzigen Loaklität in San Luis Potosí, Mexiko, bekannt ist, zeigt eine Reihe morphologischer Eigenschaften, die ilar eine Mittelstellung zwischen den beiden Untergattungen UG Roseocactus und UG Ariocarpus zuweisen; A. agavoides scheint jedoch der nächste Verwandte zu sein.

Ariocarpusbravoanus H. Hernández & E.F. Anderson sp. nov. Caulis subglobosus, ad centrum depressus, 6(-8.5)cm diametro, Tubercula ascendentia, olivacea vel cinerea-virida, triangularia, plerumque longiora quam lata, ad basin 3.7cm longa et 2cm lata. Areolae centrales, ad locum 0.8-1.4cm ab apicibus locatae, lanata. Flores subrosei-magentei,. c. 2.5cm longi. Habitatio in solo calcareo in San Luis Potosi, México. Holotypus: México, San Luis Potosí, Mpio. Guadalcázar, 1540m, matorral de Larrea, 19 Aug 1991, H. Hernández et al. 1977. (MEXU, stem in spirit). Isotypi: DES, K, MEW, MO, PATZ (stems dried and preserved in spirit, flower, seeds, and photographs). Paratypi: loc. cit., 4 Mar. 1991, H. Hernández et al. 1790 and loc. cit., 2 May 1992, H. Hernández & P. Hernández 2153 (both MEXU).

DESCRIPTION

 Stems simple, subglobose, markedly depressed centrally, to 6(--8.5)cm diam., rising no more than 2.5(-3.5)crn above ground level in habitat, with an extensive mucilage system; tubercles not crowded or basally compressed, adpressed in young individuals, becoming ascending, olivegreen or grey-green, triangular in outline, deltate in cross section, adaxially flat but becoming conspicuously concave at maturity, usually longer than broad (to 3.7cm long and 2cm wide basally), asperous at base and irregularly papillose at apex, sometimes with the papillae forming one or two lateral furrows, sharply acute to acuminate apically, and with the edges sharply acute; areoles present in the centre of the adaxial surface of the tubercles, 8-14mm from the tip, rounded to more or less elliptic, 2-5mm diam, woolly. Flowers c. 2.5cm long; outer perianth segments cream-coloured; inner perianth segments, pinkish-magenta; filaments white, anthers yellow; pistil exserted above the stamens, white; style 13mm long; stigma lobes 7, 3mm long. Seeds 1-1.5 x 0.7-1.5mm, pyriform or sacciform, black, tuberculate, with a large whitish basal hilum (Plate1),(Plate 2.1),(Plate 2.2)

DISTRIBUTION

 Known only from a very restricted area in northern San Luis Potosi, in Larrea (creosote bush) vegetation on a flat, gravelly plain of limestone origin. Associated floristic elements include Agave spp., Hechtia sp., Yucca sp., and Calliandra eriophylla.

The authors take pleasure in naming this species in honour of our friend and colleague Dra Helia Bravo Hollis, who has made such significant contributions to our knowledge of Mexican cacti.

As a result of a project to collect living and herbarium material of Mexican cacti, an interesting and new species of Ariocarpus was found in a locality north of San Luis Potosí. This new species , which is here described, possesses a combination of morphological characters not corresponding to any of the known species of this genus (Anderson 1963, 1964). Its features, in fact, place it intermediate to the subgenera Ariocarpus and Roseocactus**. Subg. Roseocactus is distinguished primarily by the presence of furrows that extend from the base to the tip of each tubercle and which are present throughout development. Species of subg. Ariocarpus tend to possess a woolly areole at or near the tip of each tubercle. The tubercles of this new species have both furrows (only in older plants) and woolly areoles, thus appearing to be intermediate between A. agavoides and A. fissuratus var. hintonii (Stuppy & Taylor 1989). As in most of the other species of Ariocarpus, the outer cell layers consist of a thin walled epidermis and a thicker-walled hypodermis (Plate 3, fig. 3). The seeds of A. bravoanus resemble those of the Ariocarpus group in shape, hilum structure and cuticular patterns of the testa (Plate 3, figs 4-9). However, in addition to differing in several features stated above, A. bravoanus is geographically separated from its closest allies by at least 75 kilometres.

After three visits to the type locality during 1991 and 1992, the senior author has been able to localize a single area where approximately 230 individuals of this species occur. Consequently, given its apparent rarity, the authors have chosen to omit publishing the precise locality to protect it from unscrupulous collectors.

The combination of diagnostic characters of A. bravoanus, namely the somewhat irregularly papillose ornamentation of the triangular tubercles, and the central, woolly areole, differentiate this species from the other members of the genus. Field observations have shown that the tubercles of A. bravoanus undergo a series of morphological modifications in the course of their development (Figs 11- 14). Younger plants usually have narrowly triangular tubercles that are 3-4 times as long as broad, adaxially flat, and suggesting those of A. agavoides. The tubercles become more compact (triangular) as they mature, their adaxial surfaces acquiring, especially at the apices, a fine, sparse, somewhat irregular papillose ornamentation (Plate 3, fig. 10); in some larger individuals lateral furrows develop. These furrows, however, are usually less evident and imperfectly formed than those of A. fissuratus. Consequently, A. bravoanus appears to be most closely related to A. agavoides by their similar circular areoles in the central portion of the tubercles and their adaxially flat, elongate, and relatively smooth tubercles. In addition, the presence of lateral furrows in the tubercles of some mature plants of A. bravoanus suggests a relationship to A. fissuratus var. hintonii.

The senior author would like to thank Veronica Alvarado, Héctor Godínez, Miguel Guevara, and Pablo Hernández for field assistance. Appreciation is also expressed to Elvia Esparza for the excellent botanical illustration (Plate 1). The research that resulted in this paper was realized through the financial assistance of the National Geographic Society (Grant 4387-90) to HMH.

REFERENCES
ANDERSON, E.F. (1963). A Revision of Ariocarpus (Cactaceae). Ill. Formal Taxonomy of the subgenus Roseocactus. In Amer. J. Bot. 50: 724-732.

ANDERSON, E.F. (1964). A Revision of Ariocarpus (Cactaceae). IV. Formal Taxonomy of the subgenus Ariocarpus. In Amer. J. Bot. 51: 144-151.

STUPPY, W. & TAYLOR, N.P. (1989). A New Variety of Ariocarpus fissuratus (Cactaceae). In Bradleya 7:84-88.

**Ed. note: The correct name for Subg. Roseocactus (A. Berger) W.T. Marshall is Ariocarpus subg. Aegopodothele Schumann, Gesamtb. Kakt., 604 (1898), typified by A. kotschoubeyanus (Lem.) Schumann.

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