136 U.S. NATIONAL MUSEUM BULLETIN 259 



underlain b}' soluble limestones. Aloreover, nowhere is the range of 

 any species in the emarginatus group transected by a ridge of non- 

 cavernous bedrock. Although the range of S. emarginatus appears 

 to be extremely disjunctive (see fig. 31), the large gap which occurs be- 

 tween northern Greenbrier Co., W. Va., and Garrett Co., Md., is best 

 accounted for by lack of adequate collecting rather than absence of 

 intermediate populations. There is a continuous stretch of cavernous 

 Mississippian limestone between the population in Garrett County and 

 the one in northern Greenbrier Coimty. Many of the caves developed 

 in this limestone, especially in northern Pocohontas County and 

 throughout most of Randolph County, have not been thoroughly in- 

 vestigated for aquatic fauna. Granted, the dispersal of S. emarginatus 

 must pass under a major drainage divide between the southward flow- 

 ing Greenbrier River and the northward flowing tributaries of the 

 Monongahela River, but this divide is highly irregular and never very 

 wide, and at least one point along its expanse is floored with Mississip- 

 pian limestone. Dispersal under a drainage divide thi-ough water 

 courses developed in different patterns from those on the adjacent 

 sm-face is strongly indicated for this species. Both Barr (1960a) and 

 Holsinger (1963) have already discussed the evidence in favor of 

 troglobite dispersal under drainage divides in limestone terrains. 



Within the range of S. gracilipes there are no problems of inter- 

 vening drainage divides. This species occupies a small part of a 

 single drainage basin (i.e., Potomac River), and while populations 

 occur on different sides of two rivers, its entire range of some 65 to 

 70 miles occurs along a continuous stretch of lower Paleozoic lime- 

 stone. Populations of S. gracilipes in southern Pennsylvania (Needy 

 Cave) and Maryland (Dam No. 4 Cave) are separated from a popula- 

 tion in West Virginia (Molers Cave) by the Potomac River, and the 

 last population is in turn separated from still a fourth population in 

 Virginia (Skyline Caverns) by the south fork of the Shenandoah River. 

 As far as can be determined, phenotypes are similar throughout the 

 range of this species, and if these rivers impose a barrier to gene flow it 

 has not yet been reflected by discernible changes in morphology. 

 There is strong implication, then, for limited dispersal through solu- 

 tion channels developed in limestones which underlie both of these 

 rivers. Although occasional dispersal across rivers by epigean 

 routes cannot be completely ruled out, the probability of this event 

 occurring is believed to be extremely low. 



It is significant that the range of S. emarginatus is also dissected by 

 a major river. The Greenbrier River cuts across the southern end 

 of this range and separates a population of S. emarginatus in Organ 

 Cave (Greenbrier Cavern System) from populations further north 

 in Court Street and Fox Caves. Little or no appreciable phenotypic 



