30 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xii. 



Holston system as a whole. — When we consider the Holston system as a unit, it is seen that 

 the headwater shells have a relatively large diameter, as the modes are at 18.5 mm. (groups 12 

 and 13). The mode for the largest shells is 20.5 mm. (group 14). Below this point in the river 

 the diameter rapidly decreases and becomes quite variable, the maxima ranging from 19.5 to 

 13.5 mm. (in groups 15 and 16); but in group 17 it again becomes relatively stable and low, 

 with a mode at 12.5 mm. The extreme downstream group increases some in diameter and has 

 a mode at 14.5 mm. (group 18). Groups 15 and 16 are thus transitional in shell diameter, be- 

 tween the headwater and downstream groups, a result of the mixture of both kinds in these 

 localities. This has been done by a mingling of the smooth and spinose kinds of shells. 



The superimposed curves show that group 14 stands with a mode at the extreme of shells 

 with great diameter, 20.5 mm., and group 17 stands at the other extreme with its mode at 

 12.5 mm.; but between these maxima occur at several intermediate points. 



A. Nollchucky, French Broad, and Tennessee Drainage. 



Group 19. This series is composed of dead shells from the site of an Indian camp near 

 Conckling, Tenn., on the Nolichucky River. They should be considered headwater shells. 

 About 75 miles above the mouth of the river, plate 47. 



The curve, plate 9, No. 19, shows that the mode occurs at 15.5 mm., and also that a second- 

 ary maximum occurs at 18.5 mm. This means a considerable relative variability for these 

 headwater shells. 



Group 20. This series is from near the mouth of the Nolichucky, plate 48. 



The curve, plate 9, No. 20, shows a mode which indicates the least diameter of shells from 

 any locality, and therefore for the genus, at 11.5 mm., and yet some individuals have very wide 

 shells. The extremely small diameter is undoubtedly due to the large proportion of young shells 

 which the group contains. The skewness of the upper side of the curve is in harmony, not only 

 with the preceding group, but also with the following one, and is therefore of significance. Had 

 it not been for the large number of young, the mode would probably have been at about 13.5 mm. 



Group 21. This group is composed of shells from three localities in the French Broad, near 

 Dandridge, about 25 miles below the mouth of the Nolichucky, plate 49. 



The curve, plate 9, No. 21, shows a mode at 16.5 mm., indicating a wider shell and greater 

 variability than in that of group 19. A secondary maximum occurs at 20.5 mm., and indicates 

 much variation. Inspection of the plate shows that the series is not homogeneous. 



Group 22. This series represents the headwater shells of the Tennessee River proper. They 

 are from Lyon Shoals below KnoxvUle, plate 50. 



The curve, plate 9, No. 22, shows a surprisingly small diameter, with a maximum at 12.5 

 to 14.5 mm. Here the youth of the shells is probably a factor in giving such a low maximum 

 to the group, because the shells are largely the narrow form turrita. In shell diameter this group 

 stands intermediate between the two Nolichucky groups, 19 and 20. 



Group 23. This series is from the Little River Shoals, about 12 miles below Knoxville, 

 Tenn. The shells are largely mature individuals, plate 51. 



The curve, plate 9, No. 23, shows the mode to occur at 18.5 mm., and its limited range sug- 

 gests relative stability. Curiously this series is not homogeneous but is composed of two forms, 

 loudonensis and turrita, a fact to be remembered throughout this discussion. 



Group 24. From Loudon, Tenn., about 40 miles below the preceding group. Large spinose 

 shells, plate 52. 



The curve, plate 9, No. 24, shows the mode to be at 17.5 mm., and a similar limited varia- 

 bility as in the preceding group ; but the upper side of the curve shows a skewness which becomes 

 more pronounced in the succeeding groups, and thus merits notice. 



Group 25. From about 40 mUes below Loudon, below the mouth of the Clinch River. Dead 

 shells from Indian shell heap, plate 53. 



The curve, plate 9, No. 25, shows a wide range of variation and the large size of the shells. 

 The maximum range from 18.5 to 21.5 mm. The secondary maximum at 15.5 mm. is signifi- 

 cant as shown by the plate, for it indicates the turrita type; the widest shells are loudonensis. 



