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ocks of Paleocene to Pliocene age. A few Cretaceousoutcrops occur in the Peten and Belize but are notextensive. The Pliocene and Miocene formationsgenerally crop out along the coast of the Peninsula,with Eocene and older rocks inland. The Sierra deTicul and most of the hill district of Campeche areformed of rocks of Eocene or Paleocene age.The Mexican part of the Yucatim Peninsula is thebest-studied area in Mexico or Central America withrespect to its invertebrate cave fauna. No caves haveyet been visited in either the Peten or northern Belize,although they are known to exist. A total of 216caves, cenotes, and other subterranean habitats havebeen biologically investigated in Campeche, QuintanaRoo, and Yucatan. Reddell (1977b) includes descriptionsof 191 of these localities. Study has been madeboth of large open-air cenotes and true caves. Theterm "cenote," though generally thought to apply toa deep, well-like cavity, may refer to either pond-likewater-floored sinkholes, deep water-floored shafts,or true caves with bodies of water in total darkness.The more open cenotes generally do not containtroglobites but may harbor species also found astroglophiles in the cavernous cenotes. Of approximately565 species known from the caves and cenotesof the Yucatan Peninsula, US are known only fromopen-air cenotes. The fauna of the Peninsula includes12 aquatic and 28 terrestrial troglobites. A generalsummary of the cavernicole fauna is in Reddell(1977b).Sierra de Bolonchen, Campeche.-The Sierra deBolonchen is located in northern Campeche immediatelysouth of the Sierra de Ticul. The region designatedby this name also includes all of the hill districtof Campeche. In much of this area there are numerousdistinct rounded hills, ranging in elevation from100 to 300 meters, and separated by large, flatbottomedvalleys up to five kilometers wide. Most ofthese valleys are dry, but some contain shallow lakes.The only prominent karst features in addition to theclosed basins are sumideros-caves which receive theseasonal flow of arroyos. Four limestone units cropout in this region: the Icaiche Formation, the Pisteand Xcabal Members of the Chichen Itza Formation,and undifferentiated rocks of Paleocene or Eoceneage. The Icaiche Formation is Paleocene or Eocenein age and is exposed only in southeastern Campecheand adjacent Guatemala. The only cave of notebelieved to be formed in the Icaiche Formation isVolcan de los Murcielagos, a large, tunnel-like cavemore than one kilometer long. Undifferentiated Eoceneor Paleocene rocks crop out in much of Campeche.Although large caves occur in these rocks inthe Sierra de Ticul, no caves of consequence havebeen found in them in Campeche. The Chichen ItzaFormation is Eocene. The Piste Member crops out inthe higher parts of the Sierra de Bolonchen south ofthe Sierra de Ticul and-continues south and east alongthe border of Campeche, Yucatan, and QuintanaRoo. The largest caves in Campeche, including largesumideros such as Grutas de Xtacumbilxunam andGrutas de San Antonio, are formed in this rock unit.The Xcabal Member crops out only in a small areabetween Champoton and Escarcega in western Campeche.One large cave, Grutas de Monte Bravo, isknown from this unit.The caves in this region are among the larger in thePeninsula and attain much greater depths than cavesin either the Sierra de Ticul or Coastal Plain. Grutasde Xtacumbilxunam with a depth of at least 105meters is the deepest cave in the Peninsula. Several ofthe caves contain deep standing pools, and at leastone (Grutas de San Jose) contains a flowing stream.The Sierra de Bolonchen is the least-known part ofthe Peninsula, but 94 species, of which 10 are troglobites,have been identified from the 18 caves studied.The troglobite fauna includes species known onlyfrom this region and species shared by other parts ofthe Peninsula. The endemic fauna includes the amphipodMayaweckelia yucatanensis Holsinger, the atyidshrimp Typhlatya campecheae Hobbs and Hobbs, thediplocentrid scorpion Diplocentrus mitchelli Francke,and the tetrablemmid spider Matta mckenziei Shear.Two species, the atyid shrimp Typhlatya pearseiCreaser and the palaemonid shrimp Creaseria morleyi(Creaser), are found throughout the Peninsula. Theremaining troglobites are known only from theCoastal Plain and the Sierra de Bolonchen. These arethe amphipod Mayaweckelia cenoticola Holsinger,the oonopid spider Oonops coecus (Chamberlin andIvie), and the pholcid spider Metagonia torete Gertsch.The only other troglobite known from the Sierra deBolonchen is an undetermined species of miIliped ofthe family Trichopolydesmidae.The troglophile fauna is generally shared by otherparts of the Yucatan Peninsula. Aquatic troglophilesinclude the physid snail Stenophysa sp., the copepodsMesocyclops ellipticus Kiefer and Paracyclops fimbriatus(Fischer), and the amphipod Hyalella azteca(Saussure). Troglophiles include the spiraxid snailsStreptostyla meridana meridana (Morelet) and S.ventricosula (Morelet); the schizomid Schizomusportoricensis (Chamberlin); the amblypygid Paraphrynusraptator (Pocock); the amaurobiid spiderGoeldia tizamina (Chamberlin and Ivie); the mysmenidspider Maymena mayana (Chamberlin andIvie); the nesticid spider Eidmannella pallida (Emerton);the oonopid spider Triaeris patellaris Bryant;56