130 U.S. NATIONAL MUSEUM BULLETIN 259 



habitats prior to fresh-water invasion. An extended transitional 

 stage is believed to have been passed in brackish waters during 

 periods of marine embayment, followed by several fresh-water 

 invasions during times of shoreline fluctuation. As seas slowly 

 receded, ancestral forms apparently already tolerant of changing 

 salinity were able to adapt gradually to fresh-water habitats, and 

 ultimately, as remnants of the sea disappeared, these forms completed 

 their migration into a fresh-water environment. Open niches newly 

 created by changing conditions near coastal areas woidd have facil- 

 itated immigration. Whether or not this postulated invasion was 

 through subterranean routes or by way of epigean waters is problem- 

 atic, but in view of present distributional data, the former route 

 would seem the more acceptable. Subsequent inland migration is 

 believed to have occurred during periods of peneplanation when 

 areas with more rugged terrains such as the Appalachians and Ozarks 

 were eroded into undulating landscapes. The most readily conceivable 

 route of inland dispersal, assuming ancestral immigrants to have been 

 phreatobitic, would have been through interstices developed in 

 alluvial deposits along broad flood plains of major rivers. 



In line with the postulated fresh water invasion of Stygonectes, one 

 might also consider the origin of subterranean Apocrangonyx and 

 Bactrurus. Both of these genera are largely interstitial and both are 

 apparently morphologicaUy closer to Stygonectes than to any other 

 crangonycid genera. While studies on these two genera are still 

 preliminary, it is significant that all data thus far accumulated on 

 their distribution patterns imply a fresh-water colonization similar 

 to that theorized for Stygonectes. 



Stygonectes is also closely allied morphologically with Stygobromus, 

 but the occurrence of the latter on both sides of the Continental 

 Divide and even in Siberia indicates a somewhat different mode of 

 fresh-water origin for this genus. Similarly, distribution patterns of 

 Synurella (holarctic) and Crangonyx s. str. (holarctic and Ethiopian) 

 differ considerably from that of Stygonectes, and for these genera one 

 is also persuaded to seek a somewhat different explanation for their 

 fresh-water origin. 



Major lines of evolution. — Three major lines of evolution mthin 

 the genus Stygonectes are indicated from recently compiled data on 

 systematics and distribution. On the basis of morphology, geographic 

 distribution, and to some extent ecology, these lines can be designated 

 and described as follows. 



1. Together, the emarginatus and spinatus species groups of the 

 central Appalachians embrace a series of well-differentiated, almost 

 exclusively cavernicolous, and apparently rare species, which, aside 

 from demonstrating significant loss and modification of certain setae 



