170 Branley A. Branson 



{Menetus) dilatatus (Gould), and Valvata species. The Yarmouthian fol- 

 lowed the Kansan glacial epoch and was the longest interglacial. During 

 that period the climate was slightly warmer than at present (Fenneman 

 1938), allowing many species to extend their ranges (Baker 1920). Later 

 investigations of Yarmouthian deposits near Evansville, Indiana (Baker 

 1920), disclosed the presence of unionid clams and various snails that no 

 longer live in streams of the Interior Low Plateau. For example, Quad- 

 rula quadrula asper (Lea 1831) now lives in streams that drain into the 

 Gulf of Mexico from Alabama to central Texas and northward to Kan- 

 sas. Another species with derivatives still present in many Low Plateau 

 streams, Amblema plicata Say, has its principal and parental stocks dis- 

 tributed from the Alabama River drainage and streams flowing into the 

 Gulf of Mexico west to central Texas and north to central Kansas. 

 Three additonal operculated snails were found in the well deposits: 

 Pleurocera unciale (Haldeman), P. alveare (Conrad), and a species of 

 Lioplax. The last two still live in the Ohio River and many of its tribu- 

 taries, whereas the first is now restricted to the upper tributaries of the 

 Tennessee River in Virginia and eastern Tennessee (Burch 1982; Good- 

 rich 1940); all have their principal relatives in the Alabama River 

 system. 



Many deposits of the previous Aftonian Interglacial and the Illino- 

 ian glacial period (Browne and Bruder 1963) indicate climatic condi- 

 tions that were considerably cooler and moister than at present. Ice 

 movement, completely overwhelming stream systems to the north, 

 caused a southward shift of faunas and, upon recession, a re-expansion 

 northward. Thus, many of the species found themselves exposed to new 

 environmental and competitive conditions that perhaps stimulated 

 extensive differentiation into species and races in extralimital areas. 

 Many gastropod species that are now restricted to more northerly lati- 

 tudes occurred in the Interior Low Plateau during Pleistocene times 

 (Browne and Bruder 1963) although some derivative species remain. 

 They include Amnicola, various species of Lymnaea, and Helisoma 

 anceps (Menke) (Branson 1972). 



The presence of the Alabama and Tennessee river system deriva- 

 tives in Interior Low Plateau streams, as far north as the Wabash River 

 of Indiana and Illinois, raises questions regarding migratory pathways 

 and strengthens a theory previously in vogue. Baker (1920) postulated 

 existence of an Appalachian river that included parts of the old Teays 

 and Tennessee rivers and their tributaries above Chattanooga and the 

 Coosa-Alabama system. Whether it is necessary to invoke the existence 

 of such a river is open to debate; other mechanisms can explain the 

 observed distributions. However, that the Alabama River system, in 

 particular the Coosa basin, has been a potent generator of species is 



