NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 77 



inclined to consider them as the extreme geographic representatives of the forms loudonensis 

 and spinosa. It looks somewhat as if lyttonensis, loudonensis, and spinosa all intergrade in this 

 region. If this is not the case their ontogeny possesses a degree of variability not known else- 

 where in these shells. 



Clinchensis. — Confined to the headwaters of the Clinch, group 6, and still abundant at St. 

 Paul, group 7, but below this locality even relatively smooth shells become spinose earlier than 

 upstream, so that by the time Clinchport is reached the mfluence of aH relatively smooth shells 

 {clinchensis and paulensis) is practically gone. At St. Paul clincJiensis has made rapid progress 

 in intergrading into paulensis. Fort Blackmore Gots 167, 168) is about the downstream limit 

 of clincJiensis, and the change to the spinose is relatively abrupt. 



Paulensis. — This form ranges from St. Paul to about Clinchport. It is transitional between 

 the smooth and spinose shells in the Clinch. This form appears to grade into the Clinchport 

 shells much as lyttonensis does into the mixed population of the lower PoweU. 



Brevis. — This shell, with rather blunt spines, is typically developed in the lower Clinch, 

 group 10, lot 17, near Kyle Ford, Tenn. Individuals, at least closely resembling them, are found 

 upstream in the vicinity of Fort Blackmore, Va., lot 164, in lot 55, from Clinchport, and down- 

 stream in the Clinch below Kyle Ford in lots 18 and 21, above the mouth of the Powell. Some 

 of the shells in the Powell near its mouth, as in lot 28, suggest this form also. With departure 

 from the vicinity of Kyle Ford this form becomes more and more obsciu'e and difficult to recog- 

 nize, so that it is practically impossible to determine its limits. In general it is bounded both 

 up and down stream by more spinose shells. 



Fluvialis. — Mainly confined to the headwaters of the North Fork of the Holston. Pro- 

 gressively downstream the undulations and nodules become more pronounced until in lot 191, 

 Mendota, Va., it grades very gradually into the form verrucosa, near the mouth of the North Fork, 

 as in lots 111 and 116, from Holston Bridge, Va. This is a very gradual transition into the down- 

 stream forms, while in the Powell and Clinch the smooth shell is rather sharply segregated 

 because the transition is more abrupt. It is interesting to observe that jiuvialis is not the pre- 

 dominant headwater form in the South Fork, but its near relative verrucosa has this range 

 In a large series of verrucosa, as in lot 94, from the South Fork, at Bluff City, there are a few 

 individuals which can not be distinguished from jiuvialis, plate 41, figures 27-31, and a few 

 similar individuals are foimd in the different lots from farther upstream at Fishdam. 



In the headwaters of the Holston proper smooth and relatively smooth shells again 

 reappear in large numbers in groups 15 and 16; but these smooth shells are not associated 

 with nodulose or undulate shells like verrucosa, as upstream in the North Fork, but with very 

 spinose individuals. The intermediate individuals are in part not only spinose toward maturity, 

 as the usual transitional individuals in groups 2 and 7 in the Powell and Clinch Rivers, but in 

 addition there is a great amount of inverse development. Therefore in the upper Holston 

 proper the gradation from the smooth shells into the spinose is very abrupt on the downstream 

 side into group 17 and upstream into group 14 it is possibly a few degrees less so. In this case 

 strong spinosity bounds the smooth shells up and down stream, a condition not found in any 

 other locality. Clearly the history of the shells in this portion of the river shows some unique 

 influence in its development. 



The smooth, or relatively smooth, jiuvialis are thus found not continuously in the upper 

 Holston drainage, as in the Powell and Clinch, but they are interrupted near the forks of the 

 Holston by the spinose shell recta. Fluvialis also outcrops throughout the range of verrucosa 

 into . which it grades imperceptibly. In the Holston the downstream limit of jiuvialis is in 

 the vicinity of Rogersville, group 16, where they end quite suddenly. 



Verrucosa. — This undulate and nodulose shell is found only in the lower part of the North 

 Fork of the Holston, from about Mendota onward to near its mouth, to the vicinity of lot 178, 

 above Rotherwood where recta abounds, and in the South Fork above Eongsport, upstream to 

 Fishdam, Tenn., where it is the predominant headwater form of shell. This is the only case 

 in the genus where a form, intermediate between the smooth and spinose kinds of shells, is 



