170 U.S. NATIONAL MUSEUM BULLETIN 259 



12, Recommendations for future study on the evolutionary biology 

 of this genus include: (a) concentrated collecting from areas where 

 large gaps are still present in the range of certain species, (b) develop- 

 ment of techniques whereby interstitial amphipods can be obtained 

 more readily, (c) experunents with pertinent species to determine 

 their toleration to varying concentrations of salt water, and (d) 

 development of more reliable and more refined methods of statistically 

 assessing the amount of morphological variation in widely ranging 

 species and utilization of these methods, if possible, to clearly delineate 

 patterns of geographic variation. 



Summary 



The systematics of the North American, subterranean amphipod 

 genus Stygonectes are revised, based on all available collections (ca. 

 300) and literature dealing with the genus. Twenty-nine species 

 are recognized and diagnosed, twenty of which (including two sub- 

 species) are newly described and five of which are completely rede- 

 scribed. The morphological affinities of Stygonectes are discussed, 

 and the genus is compared with other closely related North American 

 genera of the Crangonyx section of the family Gammaridae. The 

 genus Synpleonia is considered a synonym of Stygonectes. 



Species of Stygonectes occupy a number of distinct but closely related 

 ground-water biotopes which can be included generally under (1) caves 

 and solution channels in limestone regions and (2) interstitial habitats 

 in areas underlain by noncavernous rocks. The genus has been 

 divided into six species groups, including emarginatus, flagellatus, 

 hadenoecus, pizzinii, spinatus, and tenuis. Species are distributed 

 geographically as follows: eastern United States, including parts of 

 the Coastal Plain, Piedmont, Appalachian Valley and Ridge, and 

 Appalachian Plateau (13 species) ; south-central United States, 

 including parts of the Coastal Plain, Ozark Plateaus, Ouachitas, and 

 Osage Plains (7 species) ; and central Texas, including most of the 

 eastern Edwards Plateau region (9 species). 



The ancestral stock of Stygonectes is believed to have invaded and 

 colonized fresh-water habitats primarily during the early to middle 

 Cenozoic, when parts of eastern and southern North America were 

 covered by a shallow sea. Subsequent inland dispersal is postulated 

 to have taken place through interstitial habitats developed in flood 

 plains along major rivers. Migration into the central Appalachians 

 and the Ozark Plateau conceivably occurred during periods of pene- 

 planation, when theoretical barriers to dispersal created by elevated 



