No. 2.1 SNAILS OF THE GENTJS 10— ADAMS. 31 



Group 26. These fresh shells are of value, as they tend to corroborate the worth of the pre- 

 ceding group of dead sheUs. This series extends over the river from the preceding group to 

 Chattanooga, a distance of about 90 miles, plate 54. 



The curve, plate 9, No. 26, shows much the same range as the preceding group, and has 

 maxima at 15.5, 17.5, and 22.5 mm. But as their number is rather limited, little emphasis 

 can be placed upon them. But if both groups 25 and 26 are considered, they confirm the fact 

 of the increased diameter of the shells in this course of the river. These shells are loudonensis. 



Oroup 27. This group includes only dead shells found at old Indian camp sites, from the 

 vicinity of Bridgeport to Dodsonville, Ala., plate 55. 



The curve, plate 9, No. 27, shows a well-defined mode at 22.5 mm., and suggests relative 

 stability, although numbers of individuals are rather limited. These shells are the large indi- 

 viduals of turrita. 



Nolichucky, French Broad, and Tennessee drainage as a w^oZe.- -Taking this drainage as a 

 whole, it is seen that the modal dimensions run the entire scale from 11.5 to 22.5 mm. The 

 Nolichucky contains the smallest shells with modes at 11.5 and 15.5 mm. for groups 19 

 and 20. The single group from the French Broad shows an increase in diameter, with 16.5 mm. 

 as the mode. The secondary maximum at 20.5 mm. is probably due in the main to specimens 

 of loudonensis. The Tennessee shells show a maximum ranging from 14.5 to 12.5 mm. in group 

 22 (the immature turrita) ; and a marked increase to 18.5 mm. in group 23 (adult shells). The 

 mode drops to 17.5 mm. at Loudon in group 24, while the groups 23, 25, 26, and 27 all show 

 some increase in the diameter of the shell in both loudonensis and turrita. Group 27, from the 

 extreme lower range of the genus, exclusively turrita, appears to show greater stability than that 

 in groups 25 and 26. This series, then, shows the greatest amount of variability, a fact which 

 is not remarkable, in a way, because of the extensive area covered by it and the variety of 

 forms composing it. 



2. VARIATION IN GLOBOSITY OR SHELL INDEX. 



In addition to the diameter of the shell, the length of the aperture was measured from its 

 upper angle to the extreme tip of the canal. The diameter of the shell also gave absolute dimen- 

 sions, which were desirable for comparative pm-poses. At the same time it was desirable to 

 have some index which would give relative values. This has been secured by dividing the length 



of the aperture by the diameter of the shell : .-^ — r ^ ^ ,, = shell index. This gives a 



measure of the aperture in terms of the shell diameter and may be considered a measure of glo- 

 bosity. It should be recalled that 100 per cent means that the diameter of the shell and the 

 length of the aperture are of the same magnitude. 



a. Powell River. 



Group 1. By reference to the curve, plate 10, No. 1, it is seen that the mode occurs at 77 

 per cent globosity and that the normal form of the curve indicates relative stability. 



Group 2. The mode occurs at 75 per cent and lies almost entirely within that of group 1. 

 Group 3 modes at 77 per cent and closely parallels group 1. 



Group 4- lias its mode at 71 and 73 per cent, but its irregularities suggest greater variability. 

 Group 6 falls even lower in its mode, this occurring at 69 per cent, and it is also variable. 

 Taking the stream as a whole, it is seen that the groups fall into two distinct classes: 



(1) The first includes groups 1, 2, and 3, which have high indices and have a high degree of 

 globosity, 75 or 77 per cent, are relatively stable, and are headwater groups. 



(2) The second includes groups 4 and 5, which have relatively low indices, 69, 71, and 73 

 per cent, are relatively variable, as shown by the broad curves, and are downstream groups. 



b. Clinch River. 



Group 6. The curve, plate 11, No. 6, shows a very high index, with the mode at 83 per 

 cent. 



Group 7 has a lower modal index, at 77 per cent, and with a secondary maximum at 81 

 per cent, thus clearly showing a skewness related to the preceding group. 



17829°— 15 3 



