Studies on Mexican Cactaceae. I. Opuntia pachyrrhiza , a New Species from the Chihuahuan Desert, Mexico Hector M. Hernandez, Carlos Gomez- Hinostrosa, and Rolando T. Barcenas Departamento de Botanica, Instituto de Biologfa, UNAM, Apartado Postal 70-233, 04510 Mexico, D.F., Mexico ABSTRACT. Opuntia pachyrrhiza, a new species of Caetaeeae from the southeastern portion of the Chi¬ huahuan Desert, Mexico, is described, illustrated, and mapped. The species is easily distinguished from oth¬ erMexicanOpuntiabyitsrelativelyshortstature,its long, massive rhizome, and by the large, usually poor¬ lybranchedcladodes.Itappearstoberestrictedto the summits of La Trinidad and El Azul mountain ranges, in northern San Luis Potosf, Mexico. Resumen. Se describe, ilustra y mapea a Opuntia pachyrrhiza, una nueva especie de Cactaceae del sureste de la region del Desierto Chihuahuense, Mexico.Laespeciesedistinguefacilmentedeotros miembros mexicanos del genero por su baja esta- tura, por su rizoma largo y masivo, y por sus cla- dodiosgrandesypobrementeramificados.Sudis- tribucion se restringe aparentemente a las cimas de las Sierras de La Trinidad y El Azul, al norte de SanLuisPotosf,Mexico. Keywords:Cactaceae,endemic,Mexico,Opun¬ tia, taxonomy. Duringthe20thcenturytheCactaceaeattracted the attention of several professional botanists (Na¬ thaniel Britton and Joseph Rose, Eranz Buxbaum, Helia Bravo, David Hunt, Wilhelm Barthlott, Ed¬ ward Anderson, Beat Leuenberger, Nigel Taylor, James Mauseth, and others), who undoubtedly have producedagreatdealofscientificallyusefulinfor¬ mation on the family. In addition, due to its popu¬ larity among amateurs and private collectors, vir¬ tually hundreds of popular books, booklets, and journals on Cactaceae have been produced. The complex variation of the morphological patterns ob¬ served in several taxa within the family, in con¬ junction with the pernicious splitting tendencies andthelackofpropertypification,haveresultedin an intricate and highly unstable taxonomy. How¬ ever, despite the fact that the family has been in¬ tensively studied, information on most taxa still is scarce and fragmentary. For the last 10 years we have conducted studies on the Cactaceae, primarily from the Chihuahuan Desert Region. As a result of these, we have accumulated a great deal of basic information on many members of the family. This is the first of a series of publications in which we propose to synthesize the available knowledge, published or unpublished, about species of Mexi¬ can Cactaceae, primarily on those that are undes¬ cribed.littleknown,ortaxonomicallyconfusing. With an estimated 159 accepted species (Hunt, 1999),OpuntiaisthelargestgenusintheCactaceae and perhaps the most easily recognizable to botanists andamateurs. It has a widespreadnatural distribu¬ tion within the American continent, broadly ranging from the Peace River, in northwestern Canada (59°N), south to Patagonia, Argentina (52°S). Ac¬ cordingtoBravo(1978), Mexicocontainsthegreat¬ est diversity in the genus (117 spp.). The taxonomy of Opuntia is extremely unstable, both at the sub- generic and species levels (Britton & Rose, 1919; Bravo, 1978; Benson, 1982). This is due to its com¬ plex patterns of morphological variation, to the com¬ mon occurrence of hybrids and polyploids (e.g.. Grant & Grant, 1971; Pinkava et al., 1973; Gibson & Nobel, 1986; Baker & Pinkava, 1987), as well as to the existence of numerous horticultural forms (No¬ bel,1994).Thegenushasbeentreatedinnumerous regional floras and taxonomic treatments (e.g., Cor- rell & Johnston, 1970; Benson, 1982; Scheinvar, 1979; Pinkava, 1984; Arias et al., 1997). However, a comprehensive monograph remains to be done for the genus. Asaresultofrecentdistributionalstudiesonthe Cactaceae carried out in the southeastern portion of the Chihuahuan Desert Region in Mexico (Go- mez-Hinostrosa&Hernandez,2000;Hernandezet al., 2001), a distinct, previously undescribed spe¬ cies was discovered. Opuntia pachyrrhiza H. M. Hernandez, C. Go- mez-Hinostrosa & R. T. Barcenas, sp. nov. TYPE; Mexico. San I Aiis Potosf: Municipio Guadalcazar, Las Lagunas, a 10 km al S de San Josd de las Flores, 22°43'44"N, 100°23'02"W, alt. 2010 m, 1 1 Mar. 2000, H. M. Hernandez, C. Gomez- Hinostrosa & R. T. Barcenas 3312 (holotype, MEXU; isotypes, ASU, ENCB, IEB, MEXU [dry and spirit], MO, K, SLPM). Figure 1. Novon 11: 309-314. 2001.