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



the dominant form in the extreme headwaters. Near the mouths of both the North and South 

 Forks verrucosa grades into recta (lots 175, 178). There is thus a double series of intergrada- 

 tions between these two forms, one in each fork of the Holston. This form is relatively limited 

 in range and local differentiations have not been recognized. 



Loudonensis. — ^To understand this kind of shell it is necessary to recall that shells having 

 a similar manner of development are found in the Powell and Clinch under the respective names 

 of lyttonensis and paulensis. These three forms have the first postembryonic whorls smooth, 

 and as they grow they become spinose and normally remain so. Clearly they are related and 

 the differences appear to be those of degree. In the Holston, from Cobb Ford, lot 90, down- 

 stream, loudonensis can be clearly recognized. It is here associated with spinosa. In the 

 French Broad it continues from Dandridge, lot 136, downstream to its mouth, and from the 

 headwaters of the Tennessee proper, lot 47, downstream to Bridgeport, Ala., lot 186. It is, 

 however, far from uniform throughout this extensive range; the size of the shell and spinosity 

 increases greatly as the stream becomes larger. 



Recta. — ^This spiaose shell is found near the confluence of the North and South Forks of 

 the Holston, group 14. It is probable that it ranges upstream in the South Fork, but how far 

 is not known. In the part of the Watauga which I examined no lo were found; possibly this 

 shell is found in its lower parts. It is also found in the upper part of the Holston proper at 

 Hords Ford, lot 85, and at Currys Ford, lot 86. Certain spinose shells in lots 97 and 98 from 

 Chissolms Ford appear to be of this form, but they are not the most spinose individuals in the 

 lots, for such are apparently spinosa. Somewhat similar conditions are also found in lots 

 87, 88, from near Rogersville. 



This form is clearly related to spinosa, but they only come in contact with it in a com- 

 munity where j^wmaZis, spinosa, and recta are intermingled and intergi-ade to a remarkable degree. 



Spinosa. — In the headwaters of the Holston proper recta and spinosa are so closely related 

 that no sharp boundaries can be drawn. I am inclined to place the upstream limit of spinosa 

 at about Chissolms Ford, lots 97 and 98. Passing farther downstream by the time Cobb Ford is 

 reached (lot 90) spinosa is sufficiently free from recta, fluvialis, and intergrades, that it is the 

 predominant form of shell. By the time the vicinity of Morristown is reached spinosa reaches 

 what I have considered its typical form, lot 96. At Cobb Ford spinosa is associated with young 

 shells which are at first smooth and later are spinose. These I consider loudonensis, a form 

 with which it continues to associate to the mouth of the Holston. I have not been able to 

 recognize with certainty spinosa in the French Broad, and this seems very remarkable when we 

 consider that spinosa is very clearly related to the spinose Nolichucky shells, particularly noli- 

 cJiucTcyensis, and yet these forms are not in contact. If spinosa and nolichucky ensis were inter- 

 mingled or in close proximity in the same stream, it might seem trivial to separate them. 

 Spinosa is also closely related to the kind of turrita which occurs in the French Broad and Upper 

 Tennessee proper. It even appeai-s that spinosa and loudonensis in the lower Holston and 

 French Broad converge, because there is a variable age at which spines are developed in loudon- 

 ensis. This is a condition which recalls the relation between lyttonensis and the spinose shells 

 of the lower Powell and paulensis and the spinose shells in the vicinity of Clinchport in the 

 Clinch. The shells of group 22 from near Knoxville show how closely spinosa is related to 

 turnta in the upper Tennessee. 



TJndkensis. This spinose sheU is confined solely to the upper parts of the Nolichucky River. 

 It was found between Conkling and Limestone, Tenn. Although shells are unknown from the 

 intermediate reaches of the river downstream, yet there is no reason to think that they do not 

 grade completely into the form nolichucTcy ensis from near its mouth. This is the most spinose 

 shell found in the headwaters of the Tennessee drainage. The spinosity of unalcensis is less than 

 in the shells from farther downstream. There is a resemblance between this shell and recta which 

 is worthy of notice. 



NolichucTcy ensis and turrita. Nolichuclcyensis clearly appears to be a larger downstream 

 variation of unalcensis. It is found only in the lower part of the Nolichucky and in the French 

 Broad at the mouth of the Nolichucky; below the mouth of the Nolichucky it grades perfectly 



