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



This curve, plate 6, No. 5, is remarkable in that its mode is at 15.5 mm., and therefore the 

 same as in the headwater shells of group 1, while its skewness is on the opposite or upper side 

 of the curve, distinctly leaning toward group 4. The variabihty, as indicated by the breadth 

 of the curve, is similar to that of group 1. 



The Powell as a whole. — In considering the diameter of the Powell River shells as a unit, 

 they are remarkable for their definite relations. This is a rather variable headwater group, 

 with a mode at 15.5 mm. This indicates a small shell, and progressively downstream the diam- 

 eter increases (in groups 2 and 3) with modes at 17.5 and 18.5 mm., respectively, and finally 

 through a variable transitional series (group 4) returns, as the lower course is reached, to a mode 

 (group 5) at 15.5 mm., the same as in the headwaters, but with a skewness on the upper side 

 toward the transitional group 4. 



The range of variation of the extreme groups shows a remarkable similarity, one with a 

 left and the other with a right handed skewness. There is thus a general progressive down- 

 stream increase in the diameter or size of the shell, and some reduction in variabihty, except in 

 the extreme lower course, where there is a reduction in size and an increase of variabihty closely 

 parallehng the headwater shells. It should be recalled that the headwater shells are mainly 

 smooth and those downstream are spinose and that the most variable group (4) is a mixed 

 series of some relativel}" smooth and many spinose shells. 



b. Clinch Rlvei. 



Group 6. These sheUs are from the headwaters of the Clinch and are quite mature and large 

 shells and are smooth, plate 34. 



The curve, plate 7, No. 6, shows that the shells are of large size, with the mode at 18.5 mm., 

 and with a minor maximum at 21.5 mm. Young sheUs were relatively few in this group, and 

 this in part explains the steepness of the lower side of the curve. The skewness of the upper 

 slope, with the minor mode at 21.5 mm., shows that the shells are quite variable. These are the 

 largest shells found in the Clinch, and not the smallest as might be anticipated. 



Group 7. From about 23 miles farther downstream than the preceding group, plate 35. 



The curve, plate 7, No. 7, shows that these shellB are smaller, with a single weU-defined 

 maximum at 17.5 mm. The narrowness of the curve shows relative stability, as contrasted 

 with the variability of the preceding group, and yet these shells show great variation in spinosity 

 from a smooth to a spinose shell. 



Group 8. This group of shells is from a long segment of the river which begins about 16 

 miles below the preceding group and extends over about 14 miles of the river, plate 36. 



The curve, plate 7, No. 8, shows that the shells are narrower or smaller than those of the 

 two preceding groups, much more variable, as shown by the breadth of the curve and the maxima 

 at 16.5 and 14.5 mm. The number of immature shells in this group is a factor which has perhaps 

 influenced the lower portion of the curve. 



Group 9. This is a series of mature shells from a limited cection of the river at Clinchport, 

 Va., plate 37. 



The curve, plate 7, No. 9, shows that the shells have increased in diameter, are much more 

 stable than the preceding group, and have a mode at 17.5 mm. 



Group 10. This series is from a very restricted locality, and might be expected to be fairly 

 homogeneous. The curve, plate 7, No. 10, shows the mode at 17.5 mm., the prevailing modal 

 condition for this river, plate 38. 



Group 11. This group came from that part of the river into which the Powell empties, and 

 includes the Thirtymile Shoals. The Powell enters the Clinch about 89 miles above the mouth 

 of the Clinch, plate 39. 



The curve, plate 7, No. 11, shows the mode at 17.5 mm. with groups 7, 9, and 10, but with 

 greater variability, as shown by the breadth of the curve. The skewness of the lower side of 

 the curve with a maximum at 14.5 mm., approaches that of group 5, from the lower Powell 

 with its mode at 15.5 mm. 



The Clinch as a whole. — In considering the shells of the river as a whole, it is seen that the 

 headwater shells have the greatest diameter, 18.5 and 21.5 mm., are relatively variable, and 



