SUBTERRANEAN AMPHIPOD STYGONECTES 



63 



Diagnosis. — Differing only from *S'. t. tenuis in the absence of coxal 

 gills on pereopod 7 in both sexes and in the absence of sternal gills on 

 pleonite 1 of the male. Largest males, 16.50 mm; largest females, 

 9.00 mm. 



Variation. — Pronounced sexual dimorphism in the structure of 

 the male second antenna in S. tenuis has already been noted. The 

 regression line in figure 14 illustrates the allometric size relationship 

 between antennal lengths and standard body length in males, and 



Figure 13. — Stygonedes tenuis potomacus, new subspecies. Male (10.50 mm), Glencarlyn, 

 Va.: a, lower lip; b,c, right and left mandibles; d, maxilla 1; e, lateral sternal gill;/, 

 maxilla 2; g, maxilliped; h,i, uropods 1,2. Male (12.00 mm), Burleith Woods, Wash- 

 ington, D.C.:;, telson; k, uropod 3; /, terminal flagellar segments of antenna 2. 



