126 



U.S. NATIONAL MUSEUM BULLETIN 2 59 



typically have more restricted ranges, are more easily distinguished 

 from each other morphologically, show greater evolutionary modifi- 

 cations and specializations, and inhabit caves as often or even more 

 often than interstitial biotopes. In general, those species inhabiting 

 areas closer to present coastal regions have characters which are 

 interpreted as being phylogenetically closer to those of ancestral 

 stygonectid stock. Moreover, their broader ranges and less well- 

 defined patterns of speciation might also imply a shorter period of 

 fresh-water existence; in contrast, those species which occur farther 

 inland and have better defined patterns of speciation might have been 

 derived from an older fresh-water ancestry. 



yi^- 



i 



r 



,200 Miles. 



■IklJlllllllli^ 







Figure 30. — Distribution of Stygotiedgs in North America. [Shaded areas= regions of 

 generally contiguous distribution (i.e., greatest distance between any two localities 

 is approximately 100 miles or less). Broken line=furthest advancement of the Eocene 

 Sea during Mississippian embayment.] 



