SUBTERRANEAN AlVIPHIPOD STYGONECTES 153 



Coast (line B2). The large area between the ranges of eastern and 

 southcentral species of Stygonectes shown in figure 30 may have never 

 been mvaded and colonized by ancestral tenuis group populations, and 

 if not, it would have provided more than ample distance for extrinsic 

 isolation of the diverging stocks of lines Bi and Bo. 



Despite what appears to have been a rather lengthy separation, 

 derivative species of both lines Bi and B2 still share a number of 

 strikuig morphological similarities. In addition to the three major 

 unifying structiu^al similarities noted above for the entire tenuis group, 

 males of species of both of these phyletic lines, with few exceptions, 

 possess small distal tines on the outer apical spines of the telson. 

 Further evidence of morphological affinity is the fact that nearly all 

 species in both lines have at least some indication of palmar margin 

 concavity of the gnathopodal propods of both sexes. Males of one 

 species of line Bi (5'. tenuis) and three species of line B2 (S. barri, 

 S. alabamen^is, and *S'. balconis) show a pronounced development of 

 this character. 



iS'. tenuis, s. lat., and S. allegheniensis are closely allied morpho- 

 logically and have undoubtedly been derived from a common ancestral 

 species. These two species differ primarily in the propod pahnar 

 margins and in the size of the second antenna. In S. tenuis, palmar 

 margins are concave (especially in gnathopod 1 of the male) and the 

 second antenna is longer than the first antenna in larger males. A 

 quantitative comparison of the allometric size relationship between 

 antennal lengths and body length is shown in figures 14 and 16 for 

 S. tenuis and S. allegheniensis, respectively. Both palmar margin 

 concavity and length of the second antenna appear to have been 

 reduced or modified in S. allegheniensis, and this fact, complemented 

 by a consideration of geographic distribution, indicates that S. tenuis 

 is the more primitive of the two species and should be regarded as 

 morphologically closer to the ancestral form. 



The present ranges of S. tenuis and S. allegheniensis are allopatric 

 and physically delimited as follows: the former species occurs east 

 of the Hudson River in southern Connecticut and lower New York, 

 south of the Delaware Bay in Maryland, and southwest of the Sus- 

 quehanna River and east of the Chesapeake Bay to as far west as 

 the Blue Ridge Mountains and as far south as the James River. 

 The latter species ranges west of the Hudson River in central and 

 eastern New York and west of the Delaware River and generally 

 north and west of the Susquehanna River in Pennsylvania. With 

 the exception of a few populations in Lancaster County, this species 

 appears to be restricted to the Valley and Ridge and Appalachian 

 Plateau provinces of the Appalachian Highlands. 



