32 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [Vol.xii. 



Orowp 8 continues the progressive modal decline to 73 per cent and has a similar significant 

 skewness. 



Group 9 drops mod ally to 71 per cent, while Group 10 stands intermediately between 

 groups 8 and 9. 



Group 11 has the same mode as group 9, at 71 per cent. 



Taken as a whole, the almost uniform decline in globosity from the headwaters downstream, 

 from a mode at 83 to 71 per cent, is very remarkable for its uniformity. The position of group 

 10 is intermediate between groups 8 and 9 and is the only irregularity. This group is related 

 to the shells of the lower Powell in globosity. The headwater groups 6 and 7 show an individ- 

 uality, higher index, and greater variability than those from farther downstream, which also 

 show a similar individuality, a low index, and relative stability. 



c. Holston River System. 



Group 12. The curve, plate 11, No. 12, shows a moderately high index at 75 per cent, 

 with a skewness which recalls the curve of group 7 in the Clinch. 



Group 13 declines modally to 73 per cent and also shows a skewness toward a higher index. 



Group 14 continues the progressive modal decline to 71 per cent. 



Group 15 checks the modal decline and returns to 73 per cent and closely approximates 

 group 13 in the character of its curve. 



Group 16 also has the mode at 73 per cent, with a marked secondary maximum at 69 per 

 cent. 



Group 17 has its maximum from 71 to 73 per cent, and thus carries the decline beyond 73 

 per cent. 



Group 18 continues the decline by a double-peaked maximum at 71 per cent and at 67 

 per cent, and gives the lowest mode for the Holston. 



Considering the Holston as a whole, there is seen to be a somewhat wavering yet pro- 

 gressive and definite decline in the degree of globosity from the headwaters downstream, the 

 headwater group and the extreme downstream group representing the extremes in this respect. 

 Group 12 has a mode at 75 per cent; group 13 declines to 73 per cent; group 14 even to 71 per 

 cent; group 15 increased it to 73 per cent. Group 16 remained at 73 per cent, but had a sec- 

 ondary maximum at 69 per cent; group 17 continued the decline to 71 per cent, and group 18 

 carried it from 71 per cent down to 67 per cent. The distinctness of the curves for groups 12 

 and 18 is so marked as to be worthy of special mention. The groups as a whole appear relatively 

 variable. 



d. Nolichucky, French Broad, and Tennessee Systems. 



Groups 19, 25, and 27 are composed of dead shells which on account of injury could not 

 be used for shell index. 



Group 20, plate 13, No. 20, has a relatively low mode at 73 per cent, with a lower skewness. 



Group 21 has the highest index for this system, with a mode at 77 per cent and a secondary 

 maximum at 73 per cent. 



Group 22 declines to the opposite extreme, with a mode at 69 percent; but, as in the pre- 

 ceding group, there is a skewness toward a higher per cent, and thus these extremes meet. 



Group 23 has its mode at 75 per cent; Group 24 at 73 per cent, and Group 26 at 75 per cent, 

 with a secondary maximum at 69 per cent. 



Taking the series as a whole, at first glance there seems to be much chaos. Group 21 has 

 its mode at 77 per cent and is at one extreme, while group 22, at 69 per cent, is at the other 

 modal extreme. Between these at 73 per cent groups 20 and 24 are located and at 75 per cent 

 groups 23 and 26. The central position of group 20, between the extremes and at the point 

 of the secondary maxima, is of interest. The shells from the Tennessee River proper, groups 

 23, 24, and 26, reach their maxima at 73 or 75 per cent. 



If all groups are considered, it is clearly evident that the headwater group in the Powell, 

 Clinch, and Holston tend to be relatively globular, while the downstream groups are relatively 

 narrow or elongated. In the remainder of the groups examined, the upper part of the 



