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



Group 3 has a mode at 1 .8 mm. — a modal condition not surpassed in the Powell. 



Group 4 xinexpectedly drops in spine height to a mode at 1 .3 mm. It also shows by the maxi- 

 mum at 0.3 mm. the influence of the nodular or smooth shells. This is the most distinctly 

 transitional group in the PoweU, as its mode is intermediate between the modal extremes. 



Group 5 represents the extreme degree in spine height found in the Powell, with its mode 

 at 1.8 mm. 



Taking the stream as a whole, the groups fall into two distinct series: Those which are 

 composed primarily of smooth and low spines, not exceeding 0.8 mm. (groups 1 and 2), and those 

 which are spinose and have a mode at 1.8 mm. spine height. Group 4 is intermediate modally 

 at 1.3 mm. In general there is a progressive increase in spine height from the headwaters down- 

 stream, with the lagging increase of group 4. 



B. CLINCH RIVER. 



Group 6. The curve for this series has a mode at 0.3 mm., plate 15, No. 6. The shells were 

 practically smooth with traces of nodules. 



Group 7 also shows a mode, plate 15, No. 7, at 0.3 mm., but with a tendency toward in- 

 creasing spinosity. 



Group 8 has two maxima, one at 1.3 mm. and a similar one at 0.3 mm. 



Group 9 has greatly increased in spine height to a mode at 2.3 mm., although it almost 

 equaled the mode at 1.8 mm., and shows a progressive increase. 



Group 10 drops back to a lower mode at 1.8 mm., and a skewness toward 1.3 mm. 



Group 11 again advances to a mode very similar to that of group 9. 



As in the case of shell index, spine height shows a similar and rather uniform progressive 

 change from headwaters downstream. The headwater groups 6 and 7 are relatively spineless; 

 group 8 is a mixed series of smooth and spinose, but the spines are low, with a mode at 1.3 mm.; 

 group 10 lags with its maxima at 1.3 and 1.8 mm.; while groups 9 and 11 advance to extreme 

 spine length at 2.3 mm. 



C. HOLSTON RIVER SYSTEM. 



Group 12. This series, as shown by the curve, plate 16, No. 12, has its mode at 0.3 mm., 

 and are practically smooth shells. 



Group 13 increases in spinosity and has a mode at 0.8 mm. This is a unique condition. 



Group IJf. has greatly increased and has its maxima at 1.8 and 2.3 mm. 



Group 15 shows an anomalous curve with its mode at 0.3 mm., with a sprawling range to 

 1.8 mm. 



Group 16 shows great similarity to the preceding group and has its maxima in harmony 

 with it at 1.3 and 1.8 mm. Both of these groups have remarkable relative variability. 



Group 17 has a mode at 1.8 mm. and is intermediate between groups 13 and 14, on account 

 of its numbers at 1.3 mm. 



Group 18 is hke group 14, with its maxima at 1.8 and 2.3 mm. 



In general terms, in spite of the irregularities, there is an increase in height of spines from 

 the headwaters downstream. The most marked exception, and really a remarkable one, is in 

 the case of groups 15 and 16. They are not only anomalous for the Holston but are not equaled 

 elsewhere in their mixed character. The progressive change is shown, in group 12, with a mode 

 at 0.3 mm.; group 13, with a mode at 0.8 mm., and groups 14 and 18 with modes at 1.8 mm. 

 and almost reached a mode at 2.3 ram. Wliile group 17 has its mode also at 1.8 mm., it has a 

 large frequency at 1.3 mm. 



D. NOLICHUCKY, FRENCH BROAD, AND TENNESSEE SYSTEMS. 



Group 19. In marked contrast with the previous streams, there are no smooth headwater 

 shells. Even those farthest upstream, plate 17, No. 19, show a mode at 1.8 mm., and are thus 

 comparable with the most spiny shells from the Powell. 



Group 20. This also has the same mode as the preceding, at 1.8 mm., but the tendency 

 toward higher spines is greatly developed. 



