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Of biogeographic interest for the hadziid amphipodsis the newly extended range of Mexiweckeliafrom Cuatro Cienegas, Mexico northeastwardacross the Sierra Madre Oriental for approximately400 km to Medina Co., Texas. The present distributiontrack of this genus (see Fig. 12) extends fromnortheastern Durango, Mexico to a point justsouthwest of San Antonio in Medina Co., Texas.This track overlaps the northern part of the distributiontrack of the stygobiont stenasellid isopod Mexistenasellus(see Bowman, 1982; 1992, this vol.). Thecongruence of these tracks can be interpreted to suggestthat the evolutionary history of these two crustaceangenera was influenced by the same factors.The high number of characters shared by M. coleiin Mexico and M. hardeni in Texas suggest thatthese taxa are probably sister-species with an immediatecommon ancestor. Paralleling the closephylogenetic relationship of the amphipods is thefmding by Bowman (1992, in this vol.) that populationsof the stygobiont isopod genus Mexistenasellusfrom the same localities as the amphipods are sosimilar that they apparently represent the samespecies. There is also significant overlap of thisnorthern Mexiweckelia-Mexistenasellus track withthe northern part of the range of the stygobiontcirolanid isopod Speocirolana (see also Bowman,1992, this volume), offering further evidence for animportant generalized crustacean track fromnorth-central Mexico north and northeast intosouth-eentral Texas.Also of interest for hadziids is Holsingeriussmaragdinus, the second described species for thisgenus and the first record outside the artesian well inSan Marcos. The new locality extends the range ofthe genus westward for approximately 350 km, fromthe Edwards Plateau across the Pecos Valley to theStockton Plateau (Fig. 12). With respect to some ofits mouthparts, H. smaragdinus appears to be intermediatebetween Holsingerius and Mexiweckelia,suggesting that these two genera are closely alliedphylogenetically. This close relationship is stronglysupported by a cladistic analysis (Holsinger, inprep.), which shows that Mexiweckelia and the severalhadziid genera from the Edwards aquifer, includingboth Holsingerius and Texiweckelia,compose a nested subset of genera with a relativelyrecent common ancestry.Finally, two new localities are noted for the rare,"dwarf" genus Seborgia, marking both significantrange extensions and additional habitats for thisgroup. The new locality for S. relicta in the hyporheiczone of Hondo Creek in Medina County isthe first for this species outside the artesian well inSan Marcos and extends the range of this speciessouthwest for approximately 120 km (Fig. 11). Italso documents the occurrence of this species from ashallow groundwater (hyporheic) habitat outside thedeep phreatic waters of the Edwards Aquifer, per se.Signifying an even greater range extension forthis genus is the discovery of the closely similarspecies, Seborgia hershleri, from a spring in theDevils River gorge, approximately 250 km west ofSan Marcos (Fig. II) and just west of the Uvaldepool of the Edwards Aquifer. The morphologicalsimilarities of S. relicta from Hays and Medinacounties and S. hershleri from Val Verde County arestriking, and the few differences noted between thetwo are conceivably within the limits of geographicvariation. However, because nothing is presentlyknown about variation in the few rare species of thisgenus, and because of the apparent geographic isolationof the populations, I have elected to treat theVal Verde populations as a separate species.ACKNOWLEDGMENTSI am deeply grateful to Scott J. Harden, RobertHershler, W. Calvin Weibourn and others for collectingthe material utilized in this study. Theseworkers and James R. Reddell also provided mewith critical information on localities and habitats.A final draft of the manuscript was improved byhelpful comments from Thomas E. Bowman and JanH. Stock. I also thank Jun Zhang for inking thedrawings and helping with preparation of the figures,and the Publications and Graphics ServicesOffice at Old Dominion University for preparationof the distribution map.LITERATURE CITEDBarnard, LL., and C.M. Barnard. 1983. Freshwater Amphipodaof the World (parts 1 & II). MI. Vernon, Virginia: HayfieldAssociates. 830 pp.Barnard, LL., and G.S. Karaman. 1982. Classificatory revisionsin Gammaridean Amphipods (Crustacea), part 2. Proc. BioI.Soc. Washington, 95(1):167-187.Botosaneanu, L., and J.H. Stock. 1989. A remarkable newgenus of cavernicolous Bogidiellidae (Crustacea, Amphipoda)from Thailand. Studies in honour of Dr. PieterWagenaar Hummelinck. Foundation for Scientific Researchin Surinam and the Netherlands Antilles, Amsterdam,123: 171-184.Bousfield, E.L. 1970. Terrestrial and aquatic amphipod Crustaceafrom Rennell Island. The Natural History of RennellIsland British Solomon Islands, 6: 155-168.Bowman, T.E. 1982. Three new stenasellid isopods fromMexico (Crustacea: Asellota). Assoc. Mexican Cave Stud.Bull., 8:25-38rrexas Mem. Mus. Bull., 28:25-38.21