36 - MEMOIRS NATIONAL ACADEMY OF SCIENCES. • [vol. xn. 



Group 9 shows a great increase in the distance between spines, with its mode at 9.5 mm. 



Group 10 is very similar to the preceding and with the same mode. 



Group 11 largely coincides with the two preceding groups, but shows greater relative 

 variability. 



Considering the stream as a whole, there is the marked discontinuity between the smooth 

 and spinose shells, with group 8 as the transitional one. Groups 7 and 8 are relatively variable, 

 and groups 9, 10, and 11 relatively stable. The shells with short spines have also a short dis- 

 tance between them. From the headwaters downstream there is a general increase in the 

 distance between the spines, from spineless shells to those having modes at 6.5 mm., 6.5 to 

 7.5 mm., and on to 9.5 mm. 



C. HOLSTON RIVER SYSTEM. 



Group 12. As shown by the curve, plate 20, No. 12, these shells are practically spineless. 



Group 13 shows marked discontinuity and with two maxima, one for the relatively smooth 

 shells at 0.5 mm., and the other, the mode for spinose shells, is at 6.5 mm. 



Group IJf. shows a very wide distance between the spines, with a maximum from 10.5 mm. 

 to 11.5 mm., and thus the greatest distance yet found between the spines. 



Group 15 shows a remarkably variable group with the mode at 8.5 mm. and a spinose 

 maximum at 8.5 mm. For this group reservations must be made with regard to spinosity, 

 because of the apparent exaggeration of smoothness through the dropping of spines on the Idsi 

 whorl, while the previous ones were spinose. 



Group 16 is similar to the preceding, but the mode has declined to 7.5 mm. It is also 

 relatively variable. 



Group 17. This series very closely approximates the spinose shells of group 13 and has a 

 mode showing a distance between spines at 7.5 mm., both of which show relative stability. 



Group 18 stands somewhat transitional between the maxima at 7.5 mm. and that at 10.5 mm. 

 It has a mode at 9.5 mm. and is a variable group. 



Taking the shells of this system as a whole, considerable variability is shown. The discon- 

 tinuity between the spineless and the spinose continues, as in the preceding rivers. Another 

 marked feature is shown by the narrow distance between the groups with small spines (groups 

 13 and 17) and the greater distance between those (groups 14 and 18) with the larger spines. 

 The relatively wide distance and relative variability of groups 15 and 16 are unique. 



D. NOLICHUCKY, FRENCH BROAD, AND TENNESSEE SYSTEMS. 



Group 19. There are no spineless shells in the headwaters of this stream, the Nolichucky. 

 The modal class for distance between spines is at 7.5 mm., plate 21, No. 19, but the broad 

 maximum extends to 9.5 mm. This is a relatively variable group, as is indicated by the trun- 

 cation of the curve. 



Group 20 has a mode at the same class as the preceding, 7.5 mm., and its lower range is due 

 to its young shells. There is a minor maximum at 12.5 mm. This is also a relatively variable 

 group. 



Group 21 shows the tendency for a marked increase in the distance between spines. This 

 is due to the presence of the form loudonensis. The mode has advanced to 10.5 mm. and has a 

 broad apex at 12.5 mm. and is relatively a very variable series. 



Group 22 retains the mode reached by the last group, at 10.5 mm., and is composed of 

 young shells of turrita. 



Group 23 again carries forward spine distance to a mode at 11.5 mm. and a further tendency 

 toward increase, as shown by the maximum at 13.5 mm. 



Group 24 maintains the advance, with a mode at 13.5 mm. 



Group 25 is from the Indian shell heap and is another mixed series composed of turrita 

 and loudonensis. The mode, evidently for the turrita type of shell, is at 10.5 mm., and for 

 loudonensis is at 14.5, 15.5, and distinctly at 17.5 mm. This series is excessively variable, but 

 the numbers are too small to make the modes of much significance. The great distance between 

 the spines and the range is very remarkable. 



