160 U.S. NATIONAL MUSEUIM BULLETIN 259 



in northeastern Texas and southern Oklahoma. Subsequent isolation 

 of ancestral populations to more restricted areas within the Edwards 

 Plateau region have resulted in further genetic divergence, the details 

 of which will be discussed below. 



S. montanus is apparently restricted to ground-water biotopes in 

 the Ouachita Mountains, and to date, this species is known only from 

 a single population on Rich Mountain in Polk Co., Ark. It is likely 

 that the range of alabamensis stock once extended into the Ouachitas 

 and that extensive downcutting of this area by stream erosion since 

 the late Tertiary has resulted in extrinsic barriers between popula- 

 tions of this immediate region and those which occur further north 

 in the higher elevations of the Arkansas Valley (e.g., on Magazine 

 Mountain and in the San Bois Mountains). Populations that in- 

 habit subterranean waters of the Arkansas Valley are considered 

 conspecific with S. alabamensis s. str. 



The origin of S. barri has apparently resulted from the isolation of 

 populations of alabamensis stock to an area which lies east of the St. 

 Francois Mountains and the St. Francis River in southeastern Mis- 

 souri. Although this species differs significantly from S. alabamensis 

 in the presence of lateral spines on the telson, it is otherwise closely 

 allied morphologically. Whether the laterally spined telson repre- 

 sents a primitive character or a highly specialized one is debatable. 

 S. ozarkensis is the only other species in the tenuis group with lateral 

 spines on the telson, and for reasons given elsewhere, it is not cons- 

 sidered particularly closely related genetically to S. barri. Of possible 

 interest is the fact that at least two species in all three major evolu- 

 tionary lineages of Stygonedes possess lateral spines on the telson, but 

 the evolutionary significance of this character, if any, is not clear. 



S. alabamensis occupies a more extensive range than any other 

 species in the genus, and presently its range extends over the relatively 

 lugged topography of foiu- physiographic provinces and spans a num- 

 ber of major rivers, including the Mississippi. It is highly improbable 

 that any but the most tenuous gene flow is presently maintained 

 throughout this wide range; but, except for the limited amount of 

 morphological variation noted below, appreciable phenotypic dif- 

 ferences between individuals in the most widely separated populations 

 have not been recognized. There is, nevertheless, a tendency toward 

 loss of pertinent gill structures in certain populations, and on the 

 basis of this significant reduction, two subspecies have been delineated. 



Populations that occvu in the Osage Plains of eastern Oklahoma 

 and south to Dallas Co., Tex., show a consistent absence of coxal gills 

 on the seventh pereopods and an absence of sternal gUls on the 

 pereonites and pleonites. Populations meeting these morphological 

 criteria have been relegated to the subspecies, S. a. occidentalis. A 



