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Island, Galapagos Islands, Yucatan Peninsula) indicatesthat the Mexican species are derived from oneor more ancestral marine species.The family Palaemonidae is generally well representedin subterranean waters and six species areapparent troglobites in Mexico. Troglocubanus perezfarfanteaeVillalobos is the only Mexican member ofa genus also known from caves in Cuba. The generaCreaseria and Neopalaemon are monotypic andknown only from species inhabiting caves in theYucatan Peninsula and the Valle Nacional region ofOaxaca, respectively. Both are probably of marineorigin.The blind fish Typhliasina pearsei (Hubbs) isknown only from caves in the Yucatan Peninsula andis clearly a marine relict having as its closest relative aspecies now inhabiting marine littoral waters.Most of the marine relicts inhabiting caves inMexico are found in the vicinity of the Gulf ofMexico or Caribbean Sea. The Yucatan species aredistributed along the ancient Pliocene shoreline andhave not yet been found significantly farther inland.Other species inhabit the Sierra Madre del Sur,Chiapas-Guatemalan Highlands, and Sierra MadreOriental along their faces bordering the Gulf ofMexico. It is likely that these species have all beenderived from ancestral species inhabiting shallowwaters of the Cretaceous sea. Some may have undergonea preliminary epigean freshwater stage beforeentering caves, but this is impossible to determine.Some of the phreatobitic species probably inhabitedan interstitial habitat grading from salt through brackishto fresh water. Species of amphipod and isopodfound in the subterranean habitats of the regions ofRancho Descubridora, Durango, and Cuatro Cienegasde Carranza, Coahuila, are certainly derived fromspecies inhabiting the Cretaceous embayment thatcovered the region. Holsinger (1977a) postulates alate Cretaceous or early Tertiary age for the invasionof cave habitats in this general area.Most of the species of marine relict known fromMexico and Guatemala occur at comparatively lowelevations, but a few (such as Opisthobursa josephinae,Mexicerberus troglodytes, and various speciesof Bogidiella) are found at higher elevations. In someinstances these distributions may reflect an originalpattern of invasion as the seas receded, but in othersit seems possible that migration occurred from lowerelevations via a flooded interstitial or cavernous zone.Speocirolana pelaezi is of interest in that it rangesfrom Tamaulipas to Puebla and inland from the Sierrade Tamaulipas to the Sierra El Pino along the westernface of the Sierra Madre Oriental. Dispersal amongthese regions is now not possible and the assumptionis justifiable that its distribution reflects multipleinvasions by its ancestral species and a subsequentlack of divergence.Freshwater derivatives.-Troglobites which haveapparently evolved from freshwater ancestors includediaptomid copepods, triclad turbellarians, asellid isopods,shrimp, crayfish, crabs, and fish. All of thesespecies are closely related to species now inhabitingepigean habitats in the same general area.Three troglobitic flatworms of the family Dugesiidaeare known from caves in Mexico, and other speciesawait description. All belong to the widespreadgenus Dugesia which is found in epigean habitatsthroughout Mexico. Two species (D. typhlomexicanaMitchell and Kawakatsu and D. barbarae Mitchell andKawakatsu) are known only from caves in the Sierrade Guatemala and are closely related to the troglophilicD. guatemalensis Mitchell and Kawakatsu fromthe same region. The third species (D. mckenzieiMitchell and Kawakatsu) is known only from caves inthe San Cristobal de las Casas region, Chiapas. All ofthe described species of troglobitic Dugesia areknown from high elevation caves, but possible troglobitesare known from lower elevations.The diaptomid copepod Diaptomus (Microdiaptomus)cokeri Osorio Tafall is known only from cavesin the Sierra de El Abra, San Luis Potosi. Althoughthe subgenus is monotypic, the genus Diaptomusranges widely throughout North America.Isopods of the family Asellidae are represented inthe caves of Mexico and Guatemala by five describedand several undescribed species of the genus Caecidotea.This genus ranges throughout North America,although only one epigean species, C. communis(Say), is known from Mexico. The described troglobitesare from caves in Huehuetenango, Guatemala,and Chiapas and Veracruz, Mexico.Three troglobitic shrimp from Mexico are probablyof freshwater origin. Macrobrachium villalobosiHobbs from the Acatlan region of Oaxaca and M.acherontium Holthuis from a cave in Tabasco arealmost certainly derived directly from a freshwaterancestor, albeit one which may itself have been partlydependent on marine waters for the completion of itslife cycle as are other species of the genus. The genusBithynops is known only from one troglobite and onetroglophile in caves of the Montebello region of Chiapas.It is likely that the troglobitic B. luscus Holthuishas evolved quite recently and possibly from the troglophilicB. perspicax Holthuis, which inhabits caves ofthe same region.Three troglobitic crayfish have been describedfrom Mexican caves: Procambarus (Austrocambarus)rodriguezi Hobbs from Veracruz, P. (A.) oaxacae61