No. 2.] SNAILS OF THE GENUS 10— ADAMS. 15 



there is a great diversity among the shells within even a limited segment of a stream, and further 

 great changes are made in the streams with increasing population. In order to preserve a record 

 of such local diversity, the shells from the same shoals or the same portion of the stream (except, 

 in general, when only scattered individuals were found) were given the same "lot" number. 

 Later, after the shells had been studied, these lots were combined into "groups" for piirposes 

 of statistical comparison. Care was necessary in this grouping in order not to obscure the 

 significant details, and yet to avoid too many units for convenient comparisons. It is here 

 impossible to avoid the personal equation. Numerous supplementary lots, not used statisti- 

 cally but in other ways, are also included in this series. 



In general the "groups" are numbered consecutively, beginning with the headwaters and 

 passing downstream, the rivers being taken up in the following order: Powell, Clinch, North 

 Fork of Holston, South Fork of Holston, Holston proper, Nolichucky, French Broad, and the 

 Tennessee. The relative positions of these streams and the positions of the groups are- shown 

 on the map, plate 61. 



Unless otherwise indicated the sheUs were collected by the writer during the seasons of 

 1899, 1900, and 1901. The distances given are taken from the topographic sheets of the United 

 States Geological Survey or are from the maps or data of the United States Army Engineers. 

 For geographic details the topographic maps should be consulted. 



1. POWELL RIVER. 



Group 1. Lot 4.5. OHnger, Va. Found hving in abundance below the mill dam, where 

 the shoal contained numerous large bowlders and furnished a representative habitat for this 

 kind of snail. September 5, 1899. Although the shoals were carefuUy searched, only one speci- 

 men, lot 49, was found above the mill dam, and this was about 2 miles upstream. At Big Stone 

 Gap the North and South Forks of the Powell were examined, but no lo were found, although 

 other Pleurocerids were abundant. The water of the Powell is quite clear and the bowlders 

 are iron stained. Most of the drainage area above this point is not Hmestone but "freestone," 

 as the natives call it. The rocks are carboniferous sandstones, conglomerates, and shales. Con- 

 sult Estillville Folio, United States Geological Survey. 



These shells are the relatively smooth powellensis. Undulated and keeled shells are abun- 

 dant, but spinose shells are quite the exception. A large series consisting of several hundred 

 specimens. 



Supl. Lot 4^. From about 3 miles below OHnger, Va. A small series, six specimens of 

 fowellensis. 



Lot 41- Dryden, Va. About 2 miles up stream from the town. About 4 miles downstream 

 from lot 45, September 3, 1899. Water fairly rapid, a rocky shoal with a sHght coating of algal 

 slime. Most of the shells found in fairly rapid water, about ankle deep or a httle more. A 

 large number of small shells were found here, the largest series of young shells found in any 

 locahty. The series consists of several hundreds of specimens of powellensis. These shells are 

 shown in plate 28. 



Group 2. Supl. Lot 40- From about a mile above Lyttons Mill, near Pennington Gap, 

 Va. September 1, 1899. Four specimens, of which three are lyttonensis and one is relatively 

 smooth. 



Lot 39. Mile below Lyttons MiU, near Peimington Gap, Va. September 1, 1899. On a 

 bowldery shoal. About 10 miles below lot 41. These shells are shown in plate 29. They 

 are the form lyttonensis with a few smooth shells. If these smooth shells are powellensis, then 

 the spinose shells of lots 45 and 41 should be considered lyttonensis. 



Group 3. Lot 106. Pleasant Grove Shoal, 5 or 6 miles south of Rose Hill, Va. August 

 5, 1901. About 24 miles downstream from lot 39, near the Virginia-Tennessee line. 



Lot 180. Holiday Shoals, about 10 miles from Eose Hill, Va. Collected September 4 and 

 November 18, 1901, by John W. Pace, of Harvest, Va. Lyttonensis, with a very few examples 

 of relatively smooth shells. Many shells were slightly injured during shipment so that the series 

 is about three times larger than is indicated by the number used in the quantitative study. 



17829°— 15 2 



