SUBTERRANEAN AMPHIPOD STYGONECTES 



151 



(3 species), figure 34; southcentral United States (7 species), figure 35; 

 central Texas (4 species), figure 33. 



As already indicated, the tenuis group, along with the pizzinii 

 group, is beheved to constitute a major evolutionary lineage within 

 the genus Stygonectes. Characteristics which signify the distmctive- 

 ness of this lineage have been discussed previously. In the overall 

 structure of the gnathopods, pereopods, and abdominal side plates, 

 species of the tenuis group appear to compose a rather homogeneous 

 assemblage. A certain amount of divergence, however, is implied by 

 differences in the morphology of the telson, and these differences, 

 combined with a number of more minor ones in other structures, 

 distinguish several major phyletic branches within the group. As a 

 rule, component species of these branches occur close together geo- 

 graphically. 



On the basis of available data, which is admittedly stiU sparse in 

 several spots, I am mclined to view the phylogen}^ of the tenuis group 

 as diagramed in figure 36. Ancestral stock of this group (designated 



Figure 35. — Distribution of tenuis group of Stygonectes in south-central United States. 

 [O S. a. alabamensis; O S. a. occidentalis; ® S. barri; A S. hoiumani; X S. clantoni; 

 A 5. elatus; □ S. montanus; + S. ozarkensis. Two symbols in a circle indicate two 

 species from the same locahty.] 



