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



4. Verrucosa Reeve. 1860. This includes the nodulate and undulate shells, group 13, 

 lots 94 and 116, from the North and South Forks of the Holston River in Virginia and Tennessee. 

 They are represented by group 13, from the South Fork of the Holston, at Bluff City, Tenn. 

 The type, figured by Tryon (1873, p. 6, fig. 31), is near enough to the predominant form at 

 Bluff City and in the lower part of the North Fork, lot 91, to warrant the use of this name for 

 this form. The specimen on plate 41, figure 9, is considered a typical one. Width of shell 

 modal at 18.5 mm. and with many individuals at 19.5 and 20.5 mm., plate 8. The globosity 

 of the aperture is modal at 73 per cent and has a maximum extending to 77 per cent, plate 12. 



The mode for spine height is at 0.8 mm., plate 16. This is the only group with a maximum 

 of this dimension. The young shells, and the apical whorls of the older, are smooth or corru- 

 gated, plate 4, figures 1 to 13, and plate 41. The average height of spines is modal at 12 per 

 cent of the average distance between the spines, plate 24. The young shells are undulated. 



These shells are relatively large and heavy. They form the most distinctly uniform series 

 of the intergradations between the smooth and spinose series of shells. 



5. Lyttonensis C. C. Adams. 1914. New. This peculiar spinose shell, group 2, lot 39, 

 is from the upper Powell River, near Pennington Gap, Va. They are distinctly spinose rather 

 than corrugated, as in verrucosa. The type is shown on plate 29, figure 15. The maximum 

 for shell width stands at 17.5 mm., plate 6. This is a relatively wide shell. The globosity of 

 the aperture is modal at 75 per cent, which is a high degree, plate 10. Height of spines is 

 modal at 0.3 mm., and with many shells at 0.8 mm., plate 14. The distance between the spines 

 reaches a maximum at 7.5 mm., plate 18. The average height of spines is modal at 12 per 

 cent of the average distance between them, plate 22. The young shells are smooth or corru- 

 gated and develop spines at about the class 2 stage. 



6. Paulensis CCAdsiUXs. 1914. New. This shell is generally spinose on the last whorl only. 

 The type specimen from group 7, St. Paul, Va., lot 11, is figured on plate 35, figure 3. This 

 shell represents the transitional stage between smooth and spinose shells in the Clinch. The 

 dimensions of the shells for this form are taken from group 8, as this group is a more homogeneous 

 series than group 7 of this kind of shell. The maximum for shell width is very narrow, at 14.5 

 and 16.5 mm., plate 7. This is the narrowest series in the Clinch, while the corresponding 

 shell in the Powell, lyttonensis, is wider. The globosity of the shell is modal at 73 per cent, 

 plate 11. Spine height is modal at 1.5 mm., plate 15. The distance between the spines reaches 

 a maximum at 6.5 and 7.5 mm., plate 19. The average height of spines is modal at 17 per 

 cent of the distance between them, plate 23. The young shells are relatively smooth or undu- 

 lated and develop spinosity as a rule at about the beginning of the class 4 stage. Consult 

 plate 36 for the general appearance of this form. 



7. Recta Reeve. 1860. This is a large, rather heavy form, with spines rather than no- 

 dules. Represented abundantly by group 14, lot 178, from Kingsport, Tenn., near the con- 

 fluence of the North and South Forks of the Holston River. The shell on plate 42, figure 12, 

 is considered a typical specimen. Shells very wide, with the mode at 20.5 mm., plate 8. 

 The globosity of the shell has a maximum at 71 per cent, plate 12. This is a significant 

 reduction in globosity from that of verrucosa. Spine height has a maximum at 1.3 and 1.8 

 mm., plate 16. The apices of older shells indicate spinose young, plate 42. The average spine 

 height is modal at 17 per cent of the distance between the spines, plate 24. 



It is remarkable that this spinose form is isolated and surrounded so completely by less 

 spinose ones. Thus verrucosa bounds it upstream in both Forks of the Holston, and downstream, 

 in group 15, there are both spinose and relatively smooth shells. This is a unique condition. 



8. Brevis Anthony. 1860. This is a short, rather thick shelled form with low, blunt 

 spines. It is represented by group 10, lot 17, from Kyle Ford, Clinch River, Tenn. The typical 

 form is shown in plate 38, figure 12. Group 10 is from a restricted locality, and is fauly homo- 

 geneous, and the dimensions may be considered representative. The modal condition for 

 shell width is 17.5 mm., plate 7. The degree of globosity of the shell is 73 per cent, plate 11, 

 The height of the spine reaches a maximum at 1.8 mm., plate 15. This means very low spines. 

 The undulate form, verrucosa, has lower spines but these are very low for a distinctly spinose 



