150 U.S. NATIONAL MUSEUM BULLETIN 259 



from Gables Woods and Pequea Silver Mine have larger size means 

 than individuals in populations from the two small springs. 



These observations indicate a strong correlation between the mean 

 size of individuals in a population and the relative size of their habitat, 

 and it is concluded that the size of sexually mature amphipods of 

 S. pizzinii is affected to a large extent by the size of the habitat in 

 which they occur. This conclusion agrees with the findings of Jersche 

 (1963) on the subterranean amphipod, Niphargus tatrensis Wrzesniow- 

 ski. Data on the size of individuals of this species indicate that 

 specimens vary in size according to the geological state of their 

 habitat and in particular according to the dimensions of the sub- 

 terranean system that they inhabit. Similarly, Allee et al. (1949) 

 briefly summarized findings on snails and fish, which concluded that 

 overcrowding causes decreased growth rate of individuals and that 

 overcrowded populations are often stunted in size. 



Other than variation attributed to ecological factors, little or no 

 appreciable geographic variation has been observed in S. pizzinii. 

 Although the Susquehanna and Potomac Rivers might appear 

 prima facie as extrinsic barriers to gene flow, phenotypes do not 

 appear to differ to any extent on opposite sides of either river. The 

 single female from the well on Mineshoe Island in the Potomac River 

 offers good evidence of a potential subfiuvial dispersal route in this 

 part of the range. Above the Fall Zone the river bottom of the 

 Potomac is composed of metamorphic (crystalline) bedrock and 

 apparently provides large enough interstitial spaces for habitation b}^ 

 stygonectid amphipods. While S. pizzinii is apparently able to 

 maintain gene flow under the Potomac, collecting data indicate that 

 this species has not dispersed far westward on the Virginia side of 

 the river. With the exception of one sample of this species from a 

 "well in Virginia," for which an exact location was not given, the only 

 other available samples from west of the Potomac River are from 

 springs near the river bluff located about two miles upstream from 

 Mineshoe Island. 



The Susquehanna River, like the Potomac River, does not appear 

 to be a dispersal barrier, but unfortunately, there are no samples of 

 populations from near the river on the southwest side. The previously 

 noted 60-mile gap in the range of this species largely occurs betv/een 

 the Susquehanna River and Ellicott City, Md. Nevertheless, if this 

 river does serve as a barrier to gene flow, differences in the gene pools 

 of populations on either side of it have not yet changed to the extent 

 that recognizable phenotypic differences have developed. 



The tenuis group. — The geographic distribution of the species of 

 the tenuis group is shown regionally as follows: eastern United States 



