. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 268 Bulletin Museum of Comparative Zoology, Vol. 140, No. 6. Text-figure 1. Northern Georgia, illustrating the propinquity of the headwaters of the Etowah River (ER), of the Alabama-Coosa; of the Chattahoochee River (CR), of the Apalachicola; and of the Savannah (HSR) river systems. The topographic areas are the Appalachian Mountains (AM), Appalachian Valley (AV), and the Piedmont Plateau (PP). The Blue Ridge divide (BR) is indicated by large dots. The minor divides are indicated by smaller dots. (After Keith, 1925, p
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 268 Bulletin Museum of Comparative Zoology, Vol. 140, No. 6. Text-figure 1. Northern Georgia, illustrating the propinquity of the headwaters of the Etowah River (ER), of the Alabama-Coosa; of the Chattahoochee River (CR), of the Apalachicola; and of the Savannah (HSR) river systems. The topographic areas are the Appalachian Mountains (AM), Appalachian Valley (AV), and the Piedmont Plateau (PP). The Blue Ridge divide (BR) is indicated by large dots. The minor divides are indicated by smaller dots. (After Keith, 1925, pl. 29.) several small islands in the vicinity of present Polk County (Cooke, 1945: 273).x It is now generally agreed that this flood- ing took place in the Upper Miocene (Alt and Brooks, 1965: 408). MacNeil (1950: 98, 99) casts doubt on Cooke's evidence on the extent of Aftonian flooding, and on the basis of his identifi- cation of marine terraces concludes that the Citronelle Formation is of subaerial origin, and that there is no evidence to indicate that the sea ever transgressed it to an alti- tude of more than 150 feet during the Yarmouth interglacial stage. This flooding is now thought to have occurred during the Pliocene. MacNeil's detailed map (pl. 19) shows that even at 150 feet Peninsular Flor- 1 Orange (Ocala) Island, referred to by Clench and Turner (1950: 104), was a land mass that was separated from the continent by the Suwannee Strait dining the late Oligocene (Vanghan, 1910: 156), and its existence appears to have had no bearing on the present nnionid fauna. ida was reduced to a number of small is- lands where a unionid fauna might have had refugia. In any case, except for a small portion of the Satilla River which is above the area of maximum flooding, the remain- der of it, and all of the St. Marys River, is of more recent origin (see p. 289). It is assumed that these two rivers were mostly repopulated from the west. The ap- pearance of Elliptio c.
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