156 U.S. NATIONAL MUSEUM BULLETIN 2 59 



the absence of coxal gills on the seventh pereopods of both sexes. 

 These differences are consistent in populations on opposite sides of 

 the bay. If the Chesapeake Bay has resulted from geological events 

 during Recent time as stated by Gildersleeve (1942), then these two 

 races are probably no older than a few thousand years. 



Although individuals of S. t. tenuis are morphologically similar 

 throughout the range of this subspecies, it is problematic whether 

 populations in southern Connecticut and lower New York can still 

 exchange genes with populations in eastern Maryland. The range of 

 this subspecies is dissected by both the lower Hudson and lower 

 Delaware Rivers. This subspecies is still incompletely known on the 

 basis of only a few samples, and the large disjunction in its range 

 between southern populations and northern populations presents 

 problems that ideally can be solved by future collecting along the 

 Piedmont and Coastal Plain of Delaware and New Jersey. 



Abundant samples are available for S. t. potomacus, however, and 

 the distribution of this subspecies is more easily interpreted. The 

 Potomac River cuts across the range of S. t. potomacus, but apparently 

 does not impose a dispersal barrier. Phenotypes do not differ appre- 

 ciably on either side of the river, and a single male taken as far south 

 as near Richmond, Va., was morphologically indistinguishable from 

 specimens collected in Maryland and Washington, D.C. It is of 

 further interest to note that the population near Richmond is separated 

 from populations in Maryland and Washington not only by the 

 Potomac River but also by the Rappahannock River. 



Although a very general discussion on the ecology of S. t. potomacus 

 has been given earlier in the Systematics section, a few additional 

 remarks seem appropriate at this point, since they may help to 

 demonstrate a possible means of dispersal for this subspecies. The 

 comparative abundance of individuals and populations of S. t. poto- 

 macus in certain parts of its range can probably be attributed as much 

 to concentrated collecting efforts over an extended period of time as 

 to any other single factor. This factor is complemented, however, 

 by the characteristic abundance and shallowness of ground waters 

 in many parts of the Piedmont. An excellent account of the exten- 

 sive occurrence of ground water in the Washington, D.C. area has 

 been written by Johnston (1964). In this report, Johnston pointed 

 out that ground water in the Piedmont occurs almost exclusively in 

 crystalline rocks or in residual materials developed upon them. In 

 certain areas the overlying residual materials (mantle) are sufficiently 

 thick enough to house appreciable quantities of ground water. In 

 late winter and early spring, after snow-melt and heavy rains, points 

 of ground-water resurgence are numerous and extensive in this area. 

 Accordingly, it has been during this time of the year that the ma- 



