80 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xi:. 



since the Cretaceous, have made their start and had their greatest development ia that region, 

 and hence the limited progress which these shells have made south of Fluvialis River. 



To what degree transportation has been a factor in the history of lo can not now be 

 determined, and therefore this discussion assumes that the primary influences have been the 

 activities of the animals themselves in combination with the changes of drainage and the migra^- 

 tion of their habitats. With this preUminary understanding we may now tvirn to the dis- 

 cussion. 



Up to the close of the Cretaceous the drainage of the Upper Tennessee area was probably 

 transverse from the mountains to the northwest into the Gulf embayment. At this time 

 there appears to have been two drainage systems in the upper part of the Great Valley, one 

 which may be called the Fluvialis River, draining tlu-ough Cumberland Gap, plate 57, B, and 

 the other Loudonensis River, draining the area of the Upper Tennessee proper through the 

 Emory River route. The northernmost system or its North Fork probably contained rela- 

 tively smooth forms of lo, wliile the South Fork or the other system contained relatively spinose 

 shells. The relatively smooth primitive shells show considerable diversification in the upper 

 parts of the Great Valley, while the spinose, more specialized shells show a similar degree of 

 diversification in the area bordering this on the south. It is also probable that the smooth 

 shells were in the headwaters of the FluviaUs system, as to-day they are headwater shells, and 

 that undulations and less globular shells developed mainly lower downstream. Spinosity also 

 dechned upstream in the spinose shells, and in both systems the smaller shells were found in 

 the smaller streams and the larger shells in larger streams. At the close of the Cretaceous 

 the major differentiation within the genus had taken place and centered in two different forks 

 of one river or in two river systems. With the perfection of the Cretaceous baselevel these 

 shells, which must five in rapidly flowing water, must have occupied the upper reaches of the 

 rivers mainly, and this probably tended to isolate the lower parts of each system as much as 

 if they emptied as separate streams into salt water. 



The uplift which inaugurated the Tertiary began the destruction of the relatively bal- 

 anced condition of the drainage which developed on the Cretaceous peneplain and led to the 

 drainage changes which resulted in the diversion of the northwestward flowing streams to the 

 southwest. As the Appalacliian VaUey developed and migrated to the northeast it progressively 

 captured and diverted to the southwest the northwestward flowing streams. Thus Loudon- 

 ensis River was early diverted south and it is not unhkely that this system at an early date 

 made two important captures, one by the lower Clinch wliich pirated the upper CHnch, and 

 the other when the Nohchucky system was diverted to the southwest and its headwaters were 

 invaded by spinose shells from the South Fork of Fluviahs River, or from Loudonensis River. 

 Along the Clinch spinose sheUs were taken upstream and as the Chnch lowered its channel 

 one of its tributaries, the lower Powell, captured the upper Powell and introduced spinose 

 shells into its upper course. At this time also the forks of the Holston were probably tribu- 

 tary to the Chnch through Big Moccasin Gap, plate 59. It was probably by means of the Lower 

 Holston that the Nohchucky system was diverted to the Loudonensis system from the Fluviahs. 

 The absence of smooth shells from the Nohchucky suggests that the main South Fork of Fluviahs 

 River was not invaded by the smooth forms of lo. The peneplain and drainage adjustments 

 which produced it were probably completed late in the Eocene or Miocene. It may have been 

 late in this stage or in the Pleistocene that the Lower Holston captured the forks of the Holston 

 from Chnch River and conducted the spinose shells into the upper course of the Holston, if 

 they had not already invaded these waters from the Clinch, and if so there has been a double 

 invasion of these sheUs. 



With the diversion of the Upper Holston was completed the metamorphosis of the drainage 

 from transverse to longitudinal, from northwest to southwest, and at the same time the smooth 

 and spinose kinds of shells were intermingled and a comminghng and fusion began on a large 

 scale in each stream. The blending or interminghng appears to be most perfect where it has 

 been in progress the longest, as in the Chnch and Powell, and is most imperfect where we have 



