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U.S. NATIONAL MUSEUM BULLETIN 259 



Figure 36. — Suggested phylogeny of tenuis group of Stygonedes. 



Ao) was presumably a widely distributed brackish- water form, which 

 inhabited coastal areas during the Mississippian embayment and 

 possibly even earlier. It is impossible to assign a specific time to the 

 early bifurcation of hne Ao into the divergent stocks represented by 

 Ai and A2, but it seems reasonable to suppose that such a spht occurred 

 sometime near the beginning of the fresh-water invasion of tenuis group 

 progenitors. The subsequent splitting of line Ai probably resulted 

 from isolation of ancestral populations to fresh- water habitats adjacent 

 to the Atlantic Coast (line Bi) and corresponding isolation of other 

 ancestral populations to fresh- water habitats adjacent to the Gulf 



