SUBTERRANEAN AMPHIPOD STYGONECTES 147 



(Gildersleeve, 1942) . The Chesapeake Bay has resulted from a recent 

 dro^vning of the lower portion of the Susquehanna River, and such 

 rivers as the James, Rappahannock, and Potomac, which are now 

 separate streams, were formerly tributaries of this river (Thornbury, 

 1954). Gildersleeve (1942) attributed the formation of the Chesa- 

 peake Bay and many of the smaller drowned valleys of major streams 

 in this area to geological events in Recent time. Schuchert and Dun- 

 bar (1950) pointed out that good evidence exists for striking fluctua- 

 tions of sea levels during the Pleistocene; viz., sea levels were elevated 

 when ice sheets melted during interglacial periods and were lowered 

 when water was locked up in ice masses dm-ing glacial periods. 



The most probable time for invasion of fresh waters by ancestral 

 popidations of S. pizzinii s. str. woidd have been during the Eocene, 

 when a shallow sea inundated land areas nearly up to the present Fall 

 Zone; however, an invasion of fresh waters during the Miocene, when 

 parts of the eastern Coastal Plain w^ere again submerged, cannot be 

 ruled out. In either event, colonization of fresh-water habitats l3dng 

 adjacent to old shorelines is in line with my broader theory, which 

 was postidated to account for marine to fresh-water migration of 

 ancestral stygonectid stock in general. The present range of S. 

 pizzinii might have residted from isolation of precursor pizzinii 

 populations to a part of the Piedmont that lies to the west and north- 

 west of the present upper reaches of the Chesapeake Bay. Newly 

 established fresh-water populations of ancestral pizzinii are viewed 

 as having been isolated from their brackish water congeners when 

 shallow sea waters gradually receded eastward, following an earlier 

 transgression. Dispersal of precursor popidations to further inland 

 habitats, such as those around Lancaster, Pa., would have followed 

 later. 



The present range of S. indentatus, w^hich is restricted to a very 

 small part of the outlying Coastal Plain, is not easily explained in 

 terms of either Eocene or Miocene invasion of ancestral popidations. 

 This species inhabits shallow ground waters of an area located only 

 about 35 miles from the present Atlantic shoreline and only about 

 20 mUes from brackish waters of the lower Chesapeake Bay. This 

 area was almost certainly covered by shallow marine waters at least 

 once if not more often during interglacial periods of the Pleistocene. 

 That precursor populations of S. indentatus invaded and colonized 

 this area prior to Pleistocene time and subsequent to Miocene time 

 and were able to survive periodic inundation of their habitat by sea 

 water during interglacial periods is highly questionable. A more 

 acceptable way of accounting for the range of this species is to postulate 

 an invasion of fresh water after the last interglacial period (i.e., 

 Sangamon Interglacial) , therefore suggesting a very recent fresh- water 



