NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 39 



an unchanged spine index. Group 16 continues the decline of shell width, the globosity of the 

 shell (maximum at 69 per cent), smooth shells, and the low spines remain about stationary, 

 but the distance between the spines is reduced, and the spine index remains stationary. Group 

 17 shows extreme reduction in shell width, about a stationary condition in the globosity of the 

 shell, an increase in spine height, a relatively stationary condition with regard to the distance 

 between spines, and an increase of spine index. Group 18 is only excelled in shell width by 

 group 17, in globosity of aperture it is not excelled, it is equal to group 14, in distance between 

 spines it is only excelled by group 14, and forms the extreme limit in its high spine index. 



In general, then, the smooth headwater shells grade, group 12, into group 14, and from 

 this group downstream they again grade down to some smooth shells and rise again into the 

 very spinose shells of groups 17 and 18. The transition from relatively smooth to spinose 

 shells is made twice in this drainage, but the character of the transition is very differently 

 shown in group 13 and in groups 15 and 16. In group 13 there is true blending, while in group 

 15 and 16 it is an imperfect and confused mixture, and in marked contrast with that found in 

 the Powell and Clinch. 



The Nolichucky, French Broad, and Tennessee Rivers, considered as a whole, also show 

 considerable complexity in their sheUs. To simplify these relations the shells will be con- 

 sidered in two series, one containing shells which are spinose throughout postembryonic develop- 

 ment, as turrita and its allies (gToups 19, 20, 22, and 27), and the others which are at first smooth 

 and later become spinose, as loudonensis (in groups 21, 24, and 26). 



The allies of turrita, in the headwaters of the Nolichucky (group 19), are taken as the stand- 

 ard, and downstream, in group 20 there is a marked narrowing of the shell (due largely to young 

 shells), the character of globosity of the shell is not known, but group 20 has a mode at 73 per 

 cent; there is an increase in spine height; the distance between spines makes no significant 

 change; and the spine index shows an increase. In group 22 the width of the shells stands in- 

 termediate between groups 19 and 20, and is an increase over that of group 20; the globosity 

 of the shell shows a marked decline; both the height of spines and distance between them shows 

 a distinct increase; and the spine index is little changed, and possibly indicates a slight decline. 

 In group 27, composed of the form turrita, the shells are very wide, they are real giants; globosity 

 of the shell was not determined; spine height has greatly increased, and there is an increase in the 

 distance between the spines and in the spine index. We find, then, that from the headwaters 

 downstream there is a progressive increase in size of shell and in its spinosity. 



Turning now to the series of loudonensis, as shown in group 21, the width of the shell is 

 modal at 16.5 mm., and with a maxima at 20.5 mm.; globosity of the shell is modal at 77 per 

 cent; spine height at 2.6 mm.; distance between spines has a maxima from 9.5 to 12.5 mm.; 

 and spine index is modal at 22 per cent. From this standard, group 24 shows an increase in 

 shell width; in globosity of the shell there is a marked decline, which is in harmony with a sec- 

 ondary maxima present in group 21; the spines have elongated considerably, as has also the 

 distance between spines, and there has been an increase in spine index. In group 26 the 

 width of the shell has greatly increased; the globosity of the shell has increased, height of spines 

 has greatly increased, as has also the distance between them; and the spine index possibly 

 shows a sUght tendency toward an increase. Thus we find that from the lower French Broad 

 downstream in the Tennessee there is also a progressive increase in size and decrease in the 

 globosity of the shell and in the length of spines and the distance between them. Thus the 

 general character of the progressive changes downstream which characterize the Powell, CHnch, 

 and Holston also apphes to the remainder of the upper Tennessee drainage. Therefore in all 

 streams the quantitative data show that there is a general increase in the degree of spinosity, 

 and the length of the spines increases more rapidly than the increase in the distance between 

 them. Therefore the quantitative data show graphically and rather precisely that in all the 

 streams there is a general progressive increase in the spinosity of the shells, in their size, and a 

 reduction in their globosity downstream. The extreme forms are very far apart, but in the 

 intervening locahties the intergrading forms are found. The degree of intergradation between 

 the smooth and spinose is most completely shown in the upper parts of the Holston system. 



