No. 2.] SNAILS OF THE GENUS 10— ADAMS. 51 



With the renewed upHf t at the beginning of the Tertiary, profound changes were made in 

 adjustment to the new condition. The greatest uphft was to the northeast and the greatest 

 slope was from that direction, and consequently the drainage was destined to migrate up this 

 slope toward the New River divide. Therefore the southwestward flowing streams had in- 

 creased slopes and eroded faster than the inherited northwesterly flowing ones, which were 

 progressively diverted to the southwest. In general this process of diversion took place more 

 rapidly where the slopes were greater and at a slower rate where they were less. 



The western drainage into the embayment by the Upper and Lower Cumberland was prob- 

 ably early diverted to the south with the development of the Tertiary peneplain, the Lower 

 Cumberland-Emory Tennessee drainage axis was diverted by a tributary north of the Hiwassee 

 River. The Appalachian or Great VaUey now developed rapidly. The lower part of the 

 Clinch developed and captured the Upper Chnch below Lone Mountaui, and thus diverted it 

 from the Cumberland Gap route; by a similar method a tributary of the Lower CHnch captured 

 the Upper PoweU. The Upper Holston up to this time was probably tributary to the Chnch, 

 through Big Moccasin Gap in Clinch Mountain, but finally the Lower Holston captured the 

 upper part and led it southward by a more direct route. This was probably one of the latest 

 developments upon the Tertiary peneplain or soon after its uphft. Fragments of this old 

 peneplain are preserved at an elevation of from 1,500 to 1,600 feet. In a similar manner the 

 Lower NoHchucky, as tributary of the Lower Holston, captured the Upper Nohchucky and 

 diverted it to the southwest. In general then these diversions took place most rapidly from 

 the southwest toward the northeast, skirting the inner or concave margin of the uplift indicated 

 by Hayes and Campbell as O L, plate 57, C. Those streams were the last to be diverted which 

 were farther from this axis and to the northeast. The main diversions are the result of Tertiary 

 erosion. 



Following the Tertiary base-level, another peneplain began to the south and migrated to 

 the northeast, where at an elevation of from 1,000 to 1,100 feet it is well preserved along the 

 Upper Tennessee, the Lower Holston, French Broad, and Lower Nohchucky. The development 

 of this plain resulted in the latest adjustments of the streams just mentioned to one another, 

 particularly in the relation of the lower courses of the streams tributary to the Tennessee. 



As there are reasons for beheving that lo has developed solely within this region, and 

 maialy in the Tennessee system above Chattanooga, where these important drainage changes 

 took place duriag the Tertiary and Pleistocene, it seems that such changes must have had 

 much influence upon this group of shells. 



THE GEOGRAPHIC ORIGIN OF THE FAMILY. 



lo is confined solely to the Tennessee River system, so that its place of origiu seems to be 

 a relatively simple problem. No fossil remaius are known; therefore other lines of evidence 

 must be utilized in any attempt to solve this problem. The living members of this family, the 

 Pleuroceridse, are exclusively American. The oldest loiown fossils are from the upper Cretace- 

 ous (Laramie). It is because these are the oldest known remains that it is generaUy assumed 

 that these animals originated in the region in which the Laramie strata are found, in Colorado, 

 Wyoming, Montana, Alberta, and Saskatchewan; and further that their surviving descendents 

 have migrated to the Mississippi Valley and southeastern United States, where they now live in 

 great abundance and diversity. But the Pleuroceridas are not the only family which was 

 abundantly represented in the Laramie and which is now flourishing in southeastern United 

 States. To this group also belongs the Viviparidse and the Unionidge. 



White, who was our leading authority on the Laramie mollusca, states ('83) that this fauna 

 migrated from the West eastward. Thus, he says (pp. 483-485) : 



The great lakes which existed in western North America in the Tertiary and Laramie periods successively became 

 obliterated, but we may reasonably conclude that at least a part of the river channels of today have existed as such from 

 earlier geological tunes; that the greater part of them were established in epochs anterior to our own, and that those of 

 some of the tributaries of the present Mississippi River system are identical, at least in part, with former outlets or inlets, 

 or both, of the great ancient lakes which have just been referred to. Consequently we may reasonably conclude also 

 that the molluscan fauna of the Mississippi River s-"'stem is lineall-"^ descended from the faunae of those ancient 



