142 U.S. NATIONAL MUSEUM BULLETIN 259 



S. longipes and S. flagellatus are undoubtedly closely allied mor- 

 phologically and appear to have been split off from a common 

 progenitor. As already indicated in an earlier paper (Holsinger, 1966), 

 geographic isolation between these two closely related species could 

 have residted from a stratigraphic break in the cavernous limestone 

 situated between the population of S. longipes in eastern Kendall 

 County and the population of S. flagellatus in eastern Hays County. 

 Colonization of subterranean waters developed on either side of this 

 theoretical barrier could have resulted in isolation of populations, thus 

 influencing the subsequent development of distinct gene pools. 



To date, S. flagellatus is known only from the subterranean Purga- 

 tory Creek System, which is believed to originate in the vicinity of 

 Devil's Backbone on the divide between the Guadalupe and Blanco 

 Kivers about 14 miles northwest of San Marcos (Uhlenhuth, 1921). 

 Water, which smks into the ground in this area, presumably forms an 

 underground stream which ultimately passes through Ezells Cave and 

 flows into an aquifer which feeds the artificially dug artesian well in 

 San Marcos. S. flagellatus is known only from a few specimens pumped 

 from this well prior to 1941 and from an additional specimen coUected 

 from Ezells Cave by Kenneth Dearolf in June 1938. The absence 

 of this species from other caves in Hays County may indicate its 

 restriction to the subterranean Purgatory Creek System where it is 

 extremely rare (Holsinger, 1966). It is significant that several other 

 aquatic subterranean species are also apparently restricted to this 

 single ground-water system: a salamander, Eurycea rathhuni 

 (Stejneger) ; a shrimp, Palaemonetes antrorum Benedict; and a thermos- 

 baenacean, Monodella texana Maguii"e. The last species was only 

 recently discovered in Ezells Cave and marked a range extension of 

 the crustacean order Thermosbaenacea to the Western Hemisphere 

 (Maguire, 1964, 1965). E. rathhuni (= Typhlomolge rathhuni) has 

 been known from the artesian well in San Marcos since 1896, but its 

 systematics were only recently clarified by Mitchell and Reddell 

 (1965). A list of references to papers on P. antrorum was recently 

 prepared by ReddeU (1965). 



Early ancestral stock of the flagellatus group is believed either to 

 have invaded central Texas during the Late Cretaceous when shallow 

 inland seas were withdrawing southward or to have migrated into 

 this area during the Mississippian embayment in the early Cenozoic. 

 In either event, ancestral forms probably came to occupy a range 

 that covered an area extending along the present eastern margin of the 

 Edwards Plateau in Hays and Comal Counties and westward to 

 Kendall County. Perhaps the ancestral range was even more extensive, 

 but evidence for this is lacking. Assuming a subterranean existence 

 from the onset of fresh-water colonization, ancestral species might 



