No. 2.] 



SNAILS OF THE GENUS 10— ADAMS. 



11 



In most cases the type localities of the older authors are too inaccurate or too indefinitely 

 known to be of much value. Usually the "Holston River" or "Tennessee" represents the 

 degree of definiteness, and in a genus varying so much within a short segment of the river such 

 indefinite information is of very limited value. 



The average relations of the various forms to one another are shown in the following table : 



Table of average dimensions of the forms of lo. 



[Mazima above or below the mode are indicated by the class at which the maxima occurs, or when there is only a decided asymmetry of the curve 



this is indicated by a +.] 



Group. 



Form 

 No. 



Name. 



Shell dimensions. 



Spinosity. 



Diameter, mm. 



Globosity, per 

 cent. 



Height, mm. 



Distance between 

 spines, mm. 



Index — per 

 d 

 cent. 



- 



Mode. 



+ 





Mode. 



+ 



- 



Mode. 



+ 



- 



Mode. 



+ 



- 



Mode. 



+ 



1 

 6 

 12 

 13 

 3 



8 



14 

 10 

 18 

 19 

 20 

 21 

 24 

 27 



1 

 2 

 3 



t 



6 



7 

 8 

 9 



10 

 11 

 12 

 13 

 14 



Powellensis 



ninnhpTisis . 



+ 

 + 



15.5 

 18.5 

 18.5 

 18.5 

 17.5 

 18.5 

 14.5 



20.5 

 17.5 

 14.5 

 15.5 

 11.5 



21.5 



+ 



+ 



77 

 85 

 75 

 73 



+ 

 + 



— 



0.3 

 .3 

 .3 



.8 



- 



.5 



0.5 

 .5 

 .5 



6.5 



— 



- 



12 



12 

 12 



+ 

 + 



Fluvialis 





Lyttonensis 



16.5 



18.5 

 18.5 

 19.5 



+ 

 + 



77 

 73 



71 

 73 

 67 



73 



+ 

 + 



71 



+ 

 0.3 



+ 



1.8 

 1.5 



1.8 

 1.8 



1.8 

 1.8 

 1.8 



2.3 

 2.3 



+ 



+ 

 10.5 



8.5 

 6.5 

 7.5 

 11.5 

 9.5 

 9.5 

 7.5 

 7.5 



- 



- 



17 



+ 



Recta .... 



+ 

 12.5 



- 



17 

 17 

 17 

 22 

 17 

 22 



+ 



+ 



Brevis 





Unalrensis 



Nolichuckyensis . . . 



Loudonensis 



Turrlta 



+ 



17.5 

 22.5 



21.5 



— 



73 





— 



3.3 



2.8 





+ 



13.5 

 11.5 



= 



- 



27 

 27 



= 





1. Powellensis C. C. Adams. 1914. New. These are the smooth or slightly undulated 

 or carinated shells, group 1, lot 47, from the headwaters of the Powell River at Olinger, Va. 

 Type specimen, plate 28, figure 17. SheU width modal at 15.5 mm., plate 6. These are the 

 dwarfs or Smallest forms in the genus. The large number of young in this group have also 

 influenced the curves, as of width, plate 6. The globosity of the shell is modal at 77 per cent, 

 plate 10. The relative thinness of this form is noticeable, a trait also of young shells generally. 

 All young shells are smooth, plate 3, figures 1 and 2. 



2. Clinchensis C. C. Adams. 1914., New. These are the smooth or slightly undulated 

 shells, group 6, lot 56, from the headwaters of the Clinch River, Cleveland, Va. Type specimen, 

 plate 34, figure 13. Width of shell with a maximum ranging from 18.5 to 21.5 mm., plate 7, 

 with mode at 18.5 mm. The globosity of the aperture is modal at 83 per cent, plate 11, and 

 shows considerable variation toward a lower maximum. This shows the greatest degree of 

 globosity in the genus. Shells very thick and heavy, plate 34. Young shells are very probably 

 smooth, judging from the apices of adult shells. 



3. Fluvialis Say. 1825. These are smooth or slightly undulated shells, group 12, lot 79, 

 from the headwaters of the North Fork of the Holston River, at Saltville, Va. This is probably 

 the type locality, and plate 40, figure 24, shows a representative specimen. The shells vary 

 from smooth to those of moderate undulations, as shown in plate 40. Modal width of shell, 18.5 

 mm., plate 8. The young shells are smooth or undulated, as shown by the apices. The aperture 

 shows a high degree of globosity, with a mode at 75 per cent, plate 12, but this is less than in 

 the headwater groups in the Clinch River (groups 6 and 7), which have maxima ranging from 

 77 to 83 per cent (plates 11 and 12), while those in the Powell have a mode at 77 per cent 

 (group 1, plate 10). 



The three preceding relatively smooth forms differ so much in general appearance that 

 they can readily be distinguished by inspection. I know of no way to distinguish which of 

 these is the simplest, least specialized, or nearest the ancestral form or forms. There is con- 

 siderable individual variation in each form, and no locality is free from incipient carina, 

 undulations, or spines, at least on some of the mature shells. 



