No. 2.] • SNAILS OF THE GENUS 10— ADAMS. 35 



Group 21, with its mode at 2.8 mm., shows a marked increase in length, and the low sprawling 

 curve indicates considerable variabiUty. 



Group 22 decUnes to a mode at 2.3 mm. and appears relatively stable. This is a curve similar 

 to what we might expect from the lower Nolichucky in a series of adult shells. 



Group 23 carries the mode again forward to 2.8 mm. 



Group 24 advances the mode on to 3.3 mm. 



Group 25 has a mode at 2.8 mm. This is due to the large individuals of the form turrita, 

 and the form loudonensis. This group is based on a small series. 



Group 26 is based also upon few specimens, the mode is at 4.3 mm., but a secondary maxi- 

 mum is at 3.3 mm., which is clearly due to the young shells. 



Group 27 shows a very great decline in spine height from that of the preceding group to a 

 mode at 2.8 mm. These shells are all of the elongated form turrita. This decUne is probably 

 due to the absence of loudonensis. 



Taking these series as a whole there appears to be much confusion, but if the shells are 

 sorted into two main series, one containing chiefly shells which are spinose throughout post 

 embryonic development — such as turrita and its alhes, and the other composed of shells which 

 are at first smooth and which become spinose later — such as loudonensis, much more regularity 

 is found. We will first consider the turrita series. Beginning with groups 19 and 20 it will be 

 seen that the modes, and maxima, increase from 1.8 mm. to 2.3 mm. in group 22, and on to 2.8 

 mm., in group 27, which is composed solely of turrita. There is thus a progressive down- 

 stream increase in spine height in this series. Let us now consider the loudonensis series. These 

 shells are absent in the Nohchucky and group 21 is a mixed series (some of the turrita alUes are 

 present), but it is primarily a loudonensis population. Here the mode is at 2.8 mm. In group 

 24 the mode has increased to 3.3 mm., and in group 26 the mode is at 4.3 mm. Thus there is 

 a progressive downstream increase in the length of the spines, not only in loudonensis but also in 

 turrita and its allies, just as there was a similar change in the Powell, Chnch, and Holston Rivers. 



b. Distance between Spines. 



A. POWELL RIVER. 



Group 1. These shells were almost entirely smooth or with low spines with a short distance 

 between them, as is seen by the curve, plate 18, No. 12, No. 1, with a mode at the 0.6 mm. 



Group 2 also has its mode at the 0.5 mm. class, which includes the smooth shells, and has 

 the mode for the spinose shells at 7.5 mm: There is thus a very marked discontinuity. 



Group 3 contains only a few individuals in the smooth class, but reaches a very distinct 

 mode at 8.5 mm., which shows that the distance between the spine crests is progressively increas- 

 ing and has reached the maximal distance for the Powell as a whole. 



Group If. shows an unexpected increase in the smooth class, with a well defined maximum 

 at 0.5 mm. ; and the mode for the spinose shells is at 7.5 mm., and thus coincides with group 2. 



Group 6 again carries forward the mode to 8.5 mm., and thus has as great a distance between 

 the spines as group 3. 



Taking the PoweU as a whole, there is of course the greatest discontinuity which exists 

 between smooth and spinose shells. The mode of the smooth shells falls in the 0.5 mm. class 

 and of the spinose shells at 7.5 mm. and 8.5 mm. This shows that the distance between the 

 spines does not vary more than 1 mm. in the position of the modes. Groups 2 and 4 are the 

 most intermediate groups, as was the case in spine height. The same groups are also the most 

 variable. Group 3 is relatively the most stable, with a mode showing a distance of 8.5 mm. 

 between the spines. 



B. CLINCH RIVER. 



Group 6 is composed of spineless shells, plate 19, No. 6. 



Group 7 has its mode with the spineless or 0.5 mm. class of shells, and has a mode for 

 spinosity at 6.5 mm. for distance between spines. 



Group 8 also has its mode with the spineless shells and a mode similar to the preceding 

 group for distance between spiaes at 6.5 and 7.5 mm. Both of these curves show considerable 

 relative variation in the spinose shells. 



