SUBTERRANEAN AMPHIPOD STYGONECTES 127 



The foregoing observations would tend to place the point of origin 

 of fresh-water stygonectids in a region approximately coincident with 

 land areas formerly inundated by periodic fluctuations of the marginal 

 Cenozoic sea. From the discussions of Schuchert and Dunbar (1950) 

 one is led to conclude that both Atlantic and Gulf Coastal Plains 

 were covered periodically by brackish waters and that, depending on 

 the depth of these waters at any given time and place, estuaries, 

 swamps, inland bays, and other related features were alternately 

 commonplace throughout much of the entire area. Therefore, it 

 seems reasonable to postulate that the ancestral stock of fresh-water 

 Stygonectes previously inhabited these shallow coastal waters, passed 

 through a gradual transitional period in brackish water, and as sea 

 waters fluctuated, slowly migrated into fresh waters lying adjacent 

 to the old coasthne. Finally, as sea waters permanently receded, 

 this ancestral fauna became well established in fresh water and slowly 

 moved inland to occupy what must have been a whole series of open 

 niches newly created by the recent changes in overall environmental 

 conditions. Whether this marine to fresh-water invasion was entirely 

 through coastal interstices, partially through interstices and partially 

 thi'ough epigean waters, or totally through epigean waters is a moot 

 question. The present existence of at least two species of Stygonectes 

 in shallow phreatic waters onl}^ a few miles from the brackish water 

 of the Chesapeake Bay woidd perhaps suggest that stygonectids were 

 generally of subterranean facies from the onset of their fresh-water 

 invasion. Also pertinent is the suggestion by Vandel (1964) and other 

 Eiu^opean workers that the most probable route of fresh-water in- 

 vasion by marine ancestors of present-day hypogean gammarid 

 amphipods was through an interstitial enwonment. I do not agree 

 that all North American, subterranean amphipods (especially tro- 

 globitic species of Crangonyx s. str.) invaded continental fresh waters 

 through coastal interstices, but I am inclined to consider interstitial 

 invasion as the most acceptable pathway for Stygonectes and other 

 closely related North American genera such as Apocrangonyx and 

 Bactrurus. 



Formulation of this theory does not completely rule out the possi- 

 bility of limited marine to fresh-water invasion by ancestral stygonec- 

 tid stock during Upper Cretaceous time. During early Upper Cre- 

 taceous time. North America was inundated extensively by a shallow 

 sea (Schuchert and Dunbar, 1950; Kummel, 1961). Toward the 

 end of this period, this sea moved southward toward the Gulf of 

 Mexico (Kummel, 1961). The earliest fresh-water invasion of an 

 ancestral Stygonectes might have occurred at this time, followed later 

 in the Cenozoic by more extensive invasions. 



