SUBTERRANEAN AMPHIPOD STYGONECTES 161 



single female from a well in northern Louisiana has also been tenta- 

 tively assigned to this taxon, and its possible isolation from popula- 

 tions in the Osage Plains might imply that S. a. occidentalis is a 

 polytopic subspecies. The marginal distribution of populations in 

 the Osage Plains on the extreme western end of the range of S. ala- 

 hamensis strongly suggests that origin of this subspecies has been 

 effected through peripheral isolation. The range of S. a. occidentalis 

 is separated from that of S. a. alabamensis both by stratigraphic and 

 physiographic changes, which occur between the Osage Plains to the 

 west and the Springfield Plateau and Arkansas Valley to the east. 

 The regional climate becomes progressively di'ier westward through 

 Oklahoma, and there is a good possibility that comparatively drier 

 conditions in the eastern Osage Plains vis-a-vis those in the Ozark 

 region have forced populations of this western subspecies into deeper 

 ground-water biotopes and concomitantly placed greater limitations 

 on dispersal. 



Three other populations located on the western and southwestern 

 margins of the range of S. alabamensis s. str, also show loss of coxal 

 gills on the seventh pereopods in both sexes, but only males show loss 

 of sternal gUls on the pleonites, and both sexes still retain sternal gills 

 on the pereonites. Moreover, toward the center of the range of S. 

 alabamensis s. str. about one-half of the males examined dm'ing this 

 study were missing sternal gills on the first pleonites, but all other gills 

 structures were present. This variation, however, did not appear to 

 have a geographic pattern. Variation in the ratio of the length of the 

 first antenna to the length of the body was also noted but was more 

 pronounced in males than in females. Here again, however, there 

 was little geographic pattern to this variation except that males in 

 populations from the extreme southeastern part of the range (pri- 

 marily from east of the Mississippi River) showed a more consistent 

 trend toward proportionately longer first antennae. 



In the Ozark Plateau region, distributional patterns of S. a. ala- 

 bamensis are characterized by clusters of populations along the White 

 River and its tributaries in the south and along southern tributaries 

 to the Missouri River in the north. To date, populations have not 

 been recorded from ground waters along the major regional drainage 

 divide which occurs near the approximate center of the Ozark dome, 

 and it is problematic whether populations to the north of this divide 

 are able to exchange genes with those south of it. It is of interest to 

 note, however, that both the southern cavefish, Typhlichthys sub- 

 terraneus Girard and the Grotto Salamander, Typhlotriton spelaeus 

 Stejneger, have, according to distributional data given on the former 

 by Woods and Inger (1957) and on the latter by Brandon (1965), a 

 contiguous distribution from north to south across this divide. It 



