144 U.S. NATIONAL MUSEUM BULLETIN 259 



The occurrence of species of the jiagellatus group in apparently 

 deeper ground-water niches is, therefore, interpreted as having resulted 

 from both prolonged vertical migration and inability to compete 

 ecologically with species that have more recently invaded subterranean 

 waters of this region. Whether the rarity of species in the jiagellatus 

 group is real in terms of fewer and smaller populations or whether it 

 is only apparent because of the relative inaccessibility of deeper 

 ground-water habitats to direct observations remains unresolved. 



The hadenoecus group. — The hadenoecus group is known only on 

 the basis of a single species from the Devils Sinkhole in eastern 

 Edwards Co., Tex. The distributional relationship of this species to 

 other Texas stygonectids is shown on the map in figure 33. Aside 

 from the near equality in size of the gnathopodal propods and the 

 proportionately longer fifth pereopod, there are few structural simi- 

 larities between S. hadenoecus and species in the jiagellatus group. 

 Furthermore, with the exception of the well-developed distoposterior 

 lobes and expanded posterior margins of the pereopod bases, S. 

 hadenoecus differs morphologically from most species in the tenuis 

 group, even more than from species in the jiagellatus group. Finally, 

 the greatly expanded coxal plate of pereopod 4, the production of the 

 posterior margins of the abdominal side plates, and the particular 

 structure of the telson are characters that generally appear miique to 

 S. hadenoecus. On the basis of morphology and geographic distri- 

 bution, I have interpreted this species either as a representative of an 

 old isolated branch-off of the ancestral line leading to the jiagellatus 

 group or as a representative of a separately evolved fresh-water 

 lineage. 



S. hadenoecus probably represents an isolated, possibly relict 

 species of a group either now mostly extinct or as yet largely un- 

 discovered. The only known population of this species is found 35 

 to 40 miles farther west in the Edwards Plateau than any other sty- 

 gonectid species. This population which appears to be relatively 

 large and stable, inhabits pools located at an estimated depth of 310 

 feet below the surface. Several observations over the past few years 

 indicate that an even larger population of the troglobitic cirolanid 

 isopod, Cirolanides texensis, also occupies this habitat. The latter 

 species, however, is known from cave waters in the surrounding 

 counties of Val Verde, Uvalde, and Real and from other caves in 

 Schleicher and Hays Counties (Bowman, 1964; Reddell, 1965). 

 Distributional patterns of S. hadenoecus and C. texensis appear to 

 differ significantly, but many caves in west-central Texas remain to be 

 recorded and explored, and the chances of finding additional popula- 

 tions of S. hadenoecus or even related species do not appear altogether 

 unlikely. 



