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



ENEMIES. 



At present the most important enemy of lo is man. The contamination of streams by 

 factory waste, as at Saltville, Va., or in the Clinch by mine drainage and waste, are of a char- 

 acter which is destined to increase for some time to come. The influence of sewage, from the 

 few large cities located along the stream, has not been determined. The deforestation of the 

 mountain and the floods formed by these conditions are very injurious, as in the case of lot 112, 

 from Peltier, on the South Fork of the Holston, where hundreds of lo had been killed by the 

 floods. Further injury is caused by the action of sand and gravel, etc., upon the shoals which 

 destroys the algal food, and also changes the channels of the streams by the deposits of great 

 quantities of material eroded from the bare steep mountain slopes. Still another unfavorable 

 factor, is conditioned by lo living upon shoals. These shoals tend to be the favorite fording 

 places in a country which builds relatively few bridges. In the past, of course, there were 

 even a smaller number of bridges. The tramping of horses, mules, and the wheels of vehicles 

 injure, and must kill a large number of these shells. I have observed many shells, showing 

 repair after injury, which were found upon such shoals. I have in mind such localities as 

 Chissolms Ford and Kyle Ford, on the Holston. This form of injury is mainly in the smaller 

 rivers, because farther down stream the water is so deep that ferries are much more common. 



THE RACES AND FORMS OF 10. 



It is necessary to have some general idea of the degrees of diversity in this genus before 

 their variations can be intelligently discussed. This chapter is mainly intended to give a 

 concise idea of the most distinctly defined forms. The detailed evidence upon which these 

 distinctions are made will be discussed in later chapters. As I do not know the relative rank of 

 the different elements in the genus, I have called aU of them forms. There are so many varia- 

 tions and degrees of intei^adation, and in so many directions, that it is very difficult to dis- 

 tinguish them; and probably few persons would agree as to where the lines of demarcation 

 should be drawn. As a rule, I have tried to distinguish only those forms which are fairly 

 abundant and well defined. No effort has been made to discuss fully the quantitative data, 

 and only such results are used in describing the forms wliich appear to be from fairly homo- 

 geneous series of shells. It will be observed that the basis for the distinctions drawn have been 

 primarily based upon the character of the development of the shell and its spinosity, and that 

 the quantitative data have been used as a means of relatively concise description of the average 

 dimensions of these characteristics. 



It will be observed that only a selection of individuals are included in the forms described. 

 The shells from the lower Powell, groups 4 and 5, from the Clinch, groups 9 and 11, from the 

 Holston, groups 15, 16, 17, from the French Broad, group 21, and from the Tennessee, group 

 22, are not considered. The same is true of many supplementary lots of specimens. These 

 series are not sufficiently lai^e or homogeneous, but mixed lots of several forms, or they include 

 individuals in large numbers whose position is uncertain. This uncertainty may be due to the 

 erosion of the apical whorls, which leaves doubt as to whether the shell was smooth, undulate, 

 or spinose when young, or it may be due to the intermediate position or admixture of charac- 

 ters in the individual. The significance of some of these individuals will be discussed elsewhere. 



Particular attention should be called to the fact that, as shown by the plattings of the 

 quantitative data, none of these forms are distinctly isolated from allied forms by the 

 absence of all intei^adations. The best marked discontinuity is shown between the smooth and 

 the spinose forms, as between ^wwaZis and turrita; this is the most fundamental division within 

 the genus, and yet it is not complete. . 



I do not have access to the literature which is necessary to bring the nomenclature up to the 

 latest standard, nor have I examined the type specimens, but I have attempted to utilize the 

 information given in Tryon's ('73) monograph and have aimed to utilize as many of the old 

 names as possible. I have indicated which forms I have considered typical and all the new 

 forms are figured. The types and the representative specimens are shown on plate 1. 



