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CAVE REGIONSOnly the cave regions of Mexico, Guatemala, andBelize which have been studied with respect to theirinvertebrate fauna are discussed. The greatest emphasishas naturally been placed on the better studiedregions, and this discussion is further limited by theincomplete knowledge of the physiography andgeology.The definition of cave regions varies greatly fromarea to area. In general I have attempted to defineregions on the basis of isolation from other regions.This has been simple in some areas, particularly innorthern Mexico where isolated limestone ranges suchas Pico de Carrizal and the Sierra de Gomas are separatedby noncavernous deposits. In other areas, however,the definition of regions has been more arbitrary.An example is in southern San Luis Potosi and adjacentHidalgo and Queretaro. Despite the fact that thisarea is a rather contiguous region of limestone, fiveregions have been defined: Aquismon, Xilitla, LagunaColorado, Xilitla Plateau, and Jalapa. These regionsshow some distinct faunal differences and some differencesin elevation, geology, and vegetation. Onlywith further study and the availability of detailedgeologic and topographic maps can these and manyother regions be defined more naturally.In the description of each region I have commentedbriefly, where possible, on geology, karstmorphology, elevation, and cave type. I have alsoincluded a general discussion of the composition andrelationships of the cave fauna. The locations of thecave regions discussed below are given in Fig. 2.The discussion of physiographic provinces follows,more from convenience than conviction, the systemproposed by Raisz (1964). Other systems whichdeserve consideration include those of West (1964),Alcorta Guerrero (1966), and Russell (1969). Table 3summarizes by physiographic province and caveregion the cavernicole fauna and numher of biologicallyinvestigated caves in .\lexico, Belize, and Guatemala.Sierra Madre OccidentalThe northern portion of the Sierra Madre Occidentalconsists of a series of north-south ranges lyingbetween the ranges of the Sonoran desert and themain plateau of the Sierra. It extends from the UnitedStates into northeastern Sonora. The principalmass of the Sierra Madre Occidental is a broad highplateau formed of Tertiary volcanics extending fromnorthern Chihuahua into northern J alisco where it isbounded on the south by the Rio Santiago. The easternslopes of the Sierra tend to be gentle, but thewestern side is abrupt and cut by deep gorges, includingBarranca del Cobre, with a depth of morethan 2,000 meters. Although most of the area isformed of igneous rock, a few isolated outcrops ofCretaceous limestone have been exposed by erosionof the overlying Tertiary deposits. King (1939) hasdiscussed the geology of the northern Sierra MadreOccidental, and his geologic map shows a few of thelarger of these outcrops.Speleologically, this region is virtually unstudied.Many large shelter and shelter-like caves have beenreported, but only one has been biologically investigated.The limestone region which has been studied isa limited area near the town of Santo Tomas to thewest of Ciudad Guerrero in central Chihuahua. Alarge limestone cave is known from southern Chihuahuanear Guadalupe y Calvo but has not been visitedby speleologists.Santo Tomas, Chihuahua.-This region consists ofan isolated outcrop of Cretaceous limestone of only afew kilometers extent. Water flowing south off a lowrange of igneous hills sinks in several sumidero-typeentrances at the ends of arroyos. The water is reportedto emerge from a spring or springs a fewkilometers away. Only three caves have been investigated,the largest of which is the Socavon de SantoTomas, which attains a depth of 110 meters and alength of about 200 meters. The fauna of the regionis limited and no troglobites are known. Of the 11species thus far identified only spiders of the generaMeioneta (Linyphiidae) and Psilochorus (Pholcidae)appear to be troglophiles.Villa Matamoros. Chihuahua.-The Villa Matamorosregion is an area of Tertiary volcanics southof the city of Parra!' The only cave investigated isCueva del Salitre, a 40-meter-long passage formed inignimbrite and inhabited by a colony of cave swallows.Seven species have been identified from thecave; of these only chernetid pseudoscorpions andspiders of the genus Filistatinella (Filistatidae) aretrue cave associates.Edwards PlateauRaisz (1964) includes this area as part of the GulfCoastal Plain province, but I feel it should be includedwith the Edwards Plateau of Central Texas.It consists of a narrow band of Cretaceous limestoneex tending from a few kilometers ~orthwest of CiudadAcui'ia to the Serranias del Burro. It has been isolatedfrom the Edwards Plateau proper by the incision ofthe Rio Grande.Ciudad Acuna, Coahuila.-The only cave knownfrom this region is Cueva de los Lagos, now inundated19