154 U.S. NATIONAL MUSEUM BULLETIN 259 



Events leading to the geographic separation of populations of 

 these two species and their effect on present distributional patterns 

 are believed to have occurred rather recently. There are several 

 reasons for this conclusion: 



(1) S. tenuis, although polytypic and divisible into two sub- 

 species (i.e., S. t. tenuis and (S. t. potomacus) , has an extensive range, 

 and, as presently dehneated, its range crosses five major rivers and 

 three physiographic provinces. Several populations of S. tenuis 

 have been found in the lowland areas of the Coastal Plain just a 

 few miles from the Chesapeake Bay. These areas were covered by 

 shallow marine waters in the Eocene and Miocene and possibly for 

 short lengths of time during one or more of the interglacial periods 

 of the Pleistocene. Distributional data, then, indicate that some 

 populations of this species presently occur near brackish water and 

 were perhaps directly exposed to a shallow marine environment in 

 the not-too-distant past. 



(2) The wide range of S. allegheniensis spans three physiographic 

 provinces and crosses terrains of relatively rugged topography. 

 Apparently this Avide dispersal has taken place so recently that 

 isolation of populations in deep limestone valleys of the Appalachians 

 has not yet been completed. There is little geographic variation in 

 this species, despite its wide range. Populations, however, which 

 occur along the eastern margin of the range and occupy ground 

 waters on the eastern slopes of the Appalachian Valley and Ridge 

 province in Lebanon and Monroe Counties, Pa., and in part of the 

 Piedmont province in Lancaster County, show a few subtle but con- 

 sistent differences when compared to populations farther west. Larger 

 males of these populations have slightly concave propodal palmar 

 margins and show a tendency toward the loss of sternal gUls on the 

 first pleonites. These characters might be interpreted as intermediate 

 between S. allegheniensis and S. tenuis, particularly in regard to S. t. 

 potomacus in the case of the latter character, since males of this 

 subspecies, unlike males of S*. t. tenuis, have lost pleonite sternal gills. 

 Populations of this area are still too poorly sampled, however, to be 

 recognized incontrovertibly as representing a zone of secondary inter- 

 gradation. If hybridization is actually taldng place in this area, 

 it is best attributed to the incompleteness of the Susquehanna River 

 as an extrinsic barrier between populations of S. t. potomacus to the 

 south and populations of S. allegheniensis to the north. Although 

 future collecting may prove otherwise, another interpretation is to 

 regard these eastern marginal populations of S. allegheniensis as 

 peripheral isolates and to attribute their differences to geographic 

 variation and not to hybridization. For purposes of taxonomy, I 

 have tentatively assigned these populations to S. allegheniensis, 



