No. 2.] 



SNAILS OF THE GENUS 10— ADAMS. 29 



progressively downstream they become narrower (groups 6, 7, and 8). Farther down the diam- 

 eter again increases to 17.5 mm. (groups 9, 10, and 11), but the variability is limited in groups 

 9 and 10, and increases much in group 11. Groups 8 and 11 are the most variable and the nar- 

 rowest shells. Group 6 is smooth; group 7 is a mixture of smooth and spiaose; while the shells 

 of the remaiaing groups are largely spinose. 



c. Holston River System. 



Group 12. These smooth shells, from the type locality of lofiuvialis, are from near the head- 

 waters of the North Fork of the Holston, and are mature shells, plate 40. 



The curve, plate 8, No. 12, shows relative stability and a mode at 18.5 mm. The relative 

 absence of young shells is undoubtedly a cause of the steep slope of the lower side of the curve. 



Group 13. These are nodulose shells from the South Fork of the Holston at Bluff City, and 

 are largely mature shells, plate 41. 



The curve, plate 8, No. 13, shows a relatively variable series with a mode at 18.5 mm. Many 

 shells reach 20.5 mm., and are thus relatively large shells, and recall group 6 from the head- 

 waters of the Clinch. 



Group 14- This series is from about 20 miles downstream from the preceding group, and from 

 near the confluence of the North and South Forks of the Holston, plate 42. 



The curve, plate 8, No. 14, shows the shells to be larger than those of the preceding group 

 and with a mode at 20.5 mm., which is the greatest modal diameter of any group yet considered. 

 The skewness of the upper side of the curve at group 13 is in harmony with that of the lower 

 side of this group. In general, the group is intermediate between groups 12 and 14. 



Group 15. This series is from the headwaters of the Holston River proper, about 25 mUes 

 downstream from the preceding group. This group is composed of both smooth and spinose 

 shells, plate 43. 



The curve, plate 8, No. 15, shows a reduction from the maximum diameter of 20.5 mm., 

 reached by the mode of the preceding group, to 19.5 mm., and with two important secondary 

 maxima at 16.5 and 14.5 mm. The extreme variation indicated by these maxima is due in part 

 to immature shells, and primarily to the transitional character of the shells from smooth to 

 spinose. Even a superficial inspection of this group makes evident its remarkable complexity 

 and uniqueness. 



Group 16. This series is also composed of both smooth and spiaose shells, and is about 16 

 mUes downstream from the preceding group. From the vicinity of Rogersville, Tenn., plate 44. 



The curve, plate 8, No. 16, is about as remarkable as the preceding, and continues the 

 transitional character initiated by it. The mode is at 16.5 and 15.5 mm., with a secondary 

 maxima, at 13.5 mm., and the skewness of the upper side of this curve corresponds roughly with 

 the same tendency, though to a less degree, shown on the lower side of group 15. 



Group 17. A. series from about 14 miles downstream from the previous group. It is a 

 spinose series containing a large number of young shells, plate 45. 



The curve, plate 8, No. 17, shows a mode at 12.5 mm. This extremely small diameter is 

 remarkable. It may be due, in part, to the large number of young and spinose shells, but im- 

 maturity can hardly be the sole factor because of the definite tendency toward smaller diameter 

 already shown in the preceding upstream, groups 16, 15, and 14, and its harmony with the next 

 following downstream, group 18. The influence of the young shells may have shifted the mode 

 1 or 2 mm. smaller, but it is an undeniable fact that there has been a very marked change in the 

 character of the shells below Rogersville in this river. This group contains (with the one excep- 

 tion of group 20) the lowest mode (12.5 mm.) for the genus, even smaller than those of the 

 Powell River. 



Group 18. This series is from about 13 miles below the preceding one, and is spinose. From 

 the lower course of the Holston, near Morristown, Tenn., plate 46. 



The curve, plate 8, No. 18, shows the tendency for the downstream shells of the Holston 

 to become narrower, but not to the extreme degree shown in the preceding group. The mode 

 is at 14.5 mm., with a skewness toward increased diameter. 



