56 TJ.S. NATIONAL MUSEUM BULLETIN 25 9 



continuously with the posterior margins. Uropod 2, outer ramus 

 commonly between 2/3 to 3/4 as long as inner ramus. Apical margin 

 of telson entire, outermost apical spines often deflected laterally from 

 midUne and often possessing 1 to several small, inner, distal tines. 



Relationship.— As presently defined, the tenuis group is composed 

 of 14 species, all of which appear to be rather closely united morpho- 

 logically by at least two important characters. The greatest amount 

 of divergence occurs in the structure of the second antenna, telson, 

 and to some extent in the pereopods. On the basis of the diversity 

 in these three structures, several subgroups might be erected; how- 

 ever, these same characters tend to overlap, and until species in this 

 group are better known, splitting or subdivision will be avoided. 



As previously pointed out, the tenuis group appears to be most 

 closely allied morphologically with the pizzinii group. It can be 

 distinguished from the latter, however, as well as from other species 

 groups of Stygonectes, by the key given earlier and by the diagnosis 

 given above. 



Distribution and ecology. — The tenuis group includes nearly 

 one half of the presently recognized species of Stygonectes and spans 

 four major physiographic divisions. It ranges from central New 

 York southward through Pennsylvania to Virginia and then again 

 from eastern Alabama westward across Missouri and Arkansas to 

 eastern Kansas and southwestward to central Texas. Ecologically, 

 this group is represented in subterranean waters ranging from inter- 

 stitial habitats in sediments of the Coastal Plain and metamorphic 

 rocks of the Piedmont to a variety of cave, phreatic, and interstitial 

 habitats associated with sedimentary rocks of the Ozark and Edwards 

 Plateau regions, Ouachita Mountains, Osage Plains, and Appalachians. 



Key to Species of the tenuis Group 



1. Antenna 2 proportionately large, 75 percent as long as antenna 1 or longer 



in mature males 2 



Antenna 2 not as large, rarely if ever more than 60 percent as long as antenna 

 1 in mature males 4 



2. Antenna 2 up to 25 percent longer than antenna 1 in larger males; propod 



palmar margin of gnathopod 1 deeply concave; Piedmont and Coastal 

 Plain of eastern United States from southern Connecticut southward to 



eastern Virginia S. tenuis (Smith) 



Antenna 2 not longer than antenna 1 in larger males; propod palmar margin 

 of gnathopod 1 straight to only slightly concave 3 



3. Propod palmar margin of gnathopod 1 usually straight or only occasionally 



slightly concave; posterior margins of bases of pereopods 5-7 relatively 

 straight; telson of males with up to 12 apical spines, the outer ones often 

 with 1 or 2 small distal tines; central Appalachians from central New 

 York south to western Maryland S. allegheniensis, new species 



