146 U.S. NATIONAL MUSEUM BULLETIN 259 



is presently known only from two localities in the Coastal Plain in 

 Nansemond Co., Va. As presently delineated, the ranges of these 

 two species are separated by the Fall Zone and three major rivers. 

 Whether or not either species occupies a wider range than now indi- 

 cated by collecting records must await future biological exploration of 

 the approximately 150 mile expanse of coastal plain that lies between 

 the Washington, D.C. area to the north and the Hampton Roads 

 area to the south. On the basis of present data, however, it is assumed 

 that these two species are allopatric and isolated geographically. 

 Moreover, as mentioned in the previous section on Systematics, there 

 also appears to be some ecological differences between these two species 

 in regard to their ground-water biotopes. 



S. indentatus and S. pizzinii share a number of striking morpho- 

 logical similarities, and in view of this fact, these species appear to be 

 relatively closely related. On the other hand, this morphological 

 alliance does not necessarily infer origin of these species from an im- 

 mediate common ancestral species. As pointed out in some detail 

 below, correlation of distributional data with geological data indi- 

 cates that ancestry is probably further removed than this, and that 

 separation of the ancestral stock of these two species most likely oc- 

 curred in brackish water and not in fresh water, as might otherwise 

 be assumed. The major morphological differences between S. 

 pizzinii and S. indentatus are largely vested in the secondary sex 

 characters of the male. Notwithstanding several minor differences 

 in the gnathopocls and telson in both sexes of these species, males of 

 S. pizzinii show a modification or loss of the following major characters 

 when compared with S. indentatus: reduction in the notch of the palmar 

 margin of the first gnathopodal propod, reduction in the size and 

 setation of the fourth segment of the sixth pereopod, and loss of the 

 prominent distoposterior lobe of the basis of the seventh pereopod. 

 These changes generally fall in line with a geographic pattern which, 

 as pointed out earlier, suggests that species of Stygonectes which occur 

 further inland, by and large, show greater evolutionary modification 

 and loss of certain structures than those species that occur nearer to 

 coastal areas. 



An interpretation of the present ranges of species in the pizzinii 

 group demands consideration of geologic changes that have affected 

 coastal areas of Maryland and Virginia during the Cenozoic. Much 

 of the present Atlantic coast south of New England is, geomorphologi- 

 cally speaking, a shoreline of submergence. Its history throughout 

 most of the Cenozoic has been one of alternating transgressions and 

 regressions of shallow marine waters. The Coastal Plain of Maryland 

 and Virginia was largely inundated by sea waters during the Eocene, 

 and parts of it were again periodically submerged during the Miocene 



