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shows more than 0.5 kilometer of generally horizontalpassage (Gurnee, 1962).Only 15 species, none troglobitic, have been reportedfrom the caves of the region. The most interestingmember of the troglophile fauna is the ricinuleidCryptocellus cooki Gertsch, a species closelyrelated to C. pearsei Chamberlin from Yucatan. Theurocoptid snail Brachypodella speluncae (Pfeiffer) isalso a probable troglophile restricted to the Peb~n.The leiodid beetle Ptomaphagus (AdelopsJ barbaraePeck, tentatively identified from the region, is otherwiseknown as a troglophile in caves in Belize. Theremaining species are all also known as troglophilesin the caves of the Yucatan Peninsula and other partsof southern Mexico and adjacent Guatemala. Theseinclude the amblypygid Paraphrynus raptator (Pocock),the mysmenid spider Maymena mayana(Chamberlin and Ivie), the scytodid spider Loxoscelesyucatana Chamberlin and Ivie, the paradoxosomatidmilliped Orthomorpha coarctata (Saussure),and a pyrgodesmid milliped of the genus Synoptura.Yaxchilim, Peten, Guatemala.-This region islocated in the Peten east of the Rio Usumacinta andacross the river from the ruins of Yaxchilan in Chiapas,Mexico. Salas (1962) has studied the geology ofthe region; the caves are probably formed in theCretaceous Repasto Limestone. The three caves thathave been biologically investigated are located at elevationsof 100 to 240 meters. The work in this regionhas been done by Italian biospeleologists and most oftheir results remain unpublished. The caves aredescribed by Sbordoni et al. (1974). Two of the cavesare small, but Cueva de Juan Flores (=Cueva de Yaxchilan)contains about 840 meters of surveyed passage(McEachern, 1974).Only 13 species, none troglobitic, have thus farbeen reported from the caves of this region. Two ofthe troglophiIes, the diplurid spider Euagrus lynceusBrignoli and the oonopid spider Triaeris lacandonaBrignoli, are endemic to the region. The pyrgodesmidmilliped Calymmodesmus inquinatus Causey is alsoknown from caves in other parts of Guatemala. Theremaining troglophiles are closely allied with thefauna of other parts of Guatemala and southernMexico. These include a squamiferid isopod of thegenus Trich orhina, the schizomid Schizomus porto·ricensis (Chamherlin), the mysmenid spider Maymenamayana (Chamberlin and Ivie), the pholcid spiderPholcophora ?speophila (Chamberlin and Ivie), thescytodid spider Loxosceles yucatana Chamberlin andIvie, and a pyrgodesmid milliped of the genus Myrmecodesmus.Yucatan PeninsulaThe Yucatan Peninsula is a limestone platformprojecting northward from Central America. It isdivided politically into the Mexican states of Campeche,Quintana Roo, Yucatan, and eastern Tabasco;the Peten of Guatemala north of Flores; and Belizenorth of the Maya Mountains. Reddell (1977b) hassummarized the geology, physiography, and karstfeatures of the Mexican portion of the Peninsula.Northern Yucatan is generally low, with elevationsgradually increasing from sea level to about 30 metersin the south. The Sierra de Ticul rises abruptly fromthe southwestern Yucatan plains to elevations of 70to 100 meters. The Sierra de Bolonchen lies south ofthe Sierra de Ticul in the state of Campeche. Thenorthern part of the Sierra de Bolonchen is a distinctrange, but to the south it is broken into numerouslow hills separated by valleys, some containing pondsand lakes. This hilly region continues south into thePeten of Guatemala. The peninsular part of Belize isalso low, rolling terrain. North of the Rio Hondo,which serves as the boundary between Belize andMexico, essentially all drainage is internal. Except forthe Rio Champoton in Campeehe no streams of anyconsequence flow in the Mexican part of the Peninsula.Small lakes and ponds occur in the hilly regionsof Campeche and saline lakes are found in southernQuintana Roo, but otherwise there is not muchstanding water. The eastern Peten and Belize haveseveral sizable streams flowing into the Rio Hondo orCaribbean Sea. Parts of the Peten contain large closedbasins, some with permanent water but others, nearlyfilled with alluvium, form broad marshes during thewet season.With the exception of a few minor deposits ofgypsum and marl cropping out in southern QuintanaRoo and Campeche and alluvium on the floors of thebroader valleys in the Campeche hill district, all ofthe Mexican part of the Yucatan Peninsula is onlimestone. Limestone is also the principal rock typein the peninsular part of Belize and in the Peten ofGuatemala. It is in places covered by extensive alluvialdeposits, especially in the closed basins in thePeten, along the river valleys of the Peten and Belize,and along the Caribbean coast. Butterlin and Bonet(1963) mapped the stratigraphy of the Mexican partof the Peninsula including the extreme northern edgeof Belize and the Peten. The 1:500,000 geologic mapof Guatemala published by the Guatemala InstitutoGeografico Nacional (1975) includes the generalgeology of Belize and the Peten.Pleistocene and Holocene deposits crop out ineastern Campeche and along the northern coast ofYucatan, but most of the Peninsula is covered by55