138 U.S. NATIONAL MUSEUM BULLETIN 259 



Appalachians would have resulted in the isolation of populations 

 on either side of several major ridges and would account for present 

 distributional patterns of S. mundus and S. emarginatus. 



Despite its aberrance and retention of more spines and setae on the 

 gnathopodal propods than any other species in the emarginatus 

 group, S. stellmacki also shows a rather close morphological alliance 

 with S. emarginatus. The origin of this species, however, is still 

 unclear, and its possession of a number of characters regarded as 

 comparatively primitive might indicate an evolutionary position 

 close to the early ancestral line for the emarginatus group. The single 

 known cave record for S. stellmacki in the Nittany Valley of central 

 Pennsylvania places the range of this species approximately 65 miles 

 farther north than that of any other species in the emarginatus group. 

 If this species is considered the reUct of an earlier ancestral form rather 

 than the derivative of a more recent progenitor, its present range might 

 offer a clue to the earlier distribution of the emarginatus group. 



Aside from notable divergence in the shape of pereopod bases, S. 

 cooperi and S. morrisoni appear to be more closely related to each 

 other than either is to any other species in the emarginatus subgroup. 

 The presence of only simple, lateral sternal gills (as opposed to 

 bifurcated ones) in S. cooperi poses some interesting problems, since 

 this is the only stygonectid species outside of central Texas that has 

 undergone a reduction in this structure. There is presently no good 

 explanation for the modification of lateral sternal gills, but the fact 

 that this reduction occurs in species occupying widely separated 

 geographical areas and belonging to divergent evolutionary lineages 

 is more easily attributed to parallelism than to close afl&nity. Both 

 S. cooperi and S. morrisoni are extremely rare species and are known 

 only from single caves located in well-isolated bands of Devonian 

 limestone. Furthermore, their ranges are separated by several noncav- 

 ernous ridges and a distance of more than 100 miles. S. morrisoni 

 also occurs syntopicaUy with S. mundus in Witheros Cave, Bath Co., 

 Va. Assuming allopatric speciation, occupancy of the same habitat 

 by two species closely enough related to each other to be assigned to 

 the same species subgroup suggests that ground waters in the vicinity 

 of Witheros Cave have been invaded at least twice by species derived 

 from a common ancestral form. 



The spinatus group. — The restricted range of S. spinatus, pres- 

 ently the only known species of the spinatus group, is shown in figure 

 31. The heavily setose posterior margins of the abdominal side plates 

 and the expanded posterior margins of the pereopod bases of this rare 

 species are both regarded as primitive characters. On the other hand, 

 the loss of pertinent spines and setae on the gnathopodal propods and 

 the loss or great reduction of the umer lobes of the lower lip are both 



