No. 2.] SNAILS OF THE GENUS 10— ADAMS. 81 



reason to think that it is of relatively late origin, as in the Upper Holston near Rogersville, 

 where inversion runs riot and crossing has been possible on a large scale. 



The uplift at the beginning of the Pleistocene continued the active migration of the rapid- 

 water habitat. The drainage responses to the new conditions again began in the southwest 

 and migrated to the northeast, but mainly on account of the relatively short time which has 

 elapsed since the initiation of this cycle the peneplain has only progressed, in a well-developed 

 condition, up the Great VaUey to the vicinity of the headwaters of the Tennessee proper. It 

 is also probable that at this time the lower parts of the Holston, French Broad, and Nohchucky 

 developed their present relations to the Tennessee proper. The French Broad probably cap- 

 tured the Nohchucky from the Lower Holston, and some tributary of the Nohchucky may even 

 have been captured by the South Fork of the Holston, and thus introduced the spinose recta 

 if it is not the result of transportation. 



But beyond the broad plain there developed by erosion, near the head of the Tennessee 

 proper, extended valley tongues for some distance up the other rivers and up the Holston possibly 

 to the vicinity of Rogersville, Tenn., and this suggests that it was probably at this relatively 

 late date that the forks of the Holston were diverted to the southward. Of course during all 

 this time the headwaters of all the tributaries of the Tennessee were advancing toward the 

 divides, and in this way the shells have had their range extended where lime-bearing rocks have 

 permitted it. By this method of elongation of the habitat its extent has increased and the 

 animals at the extremities of their range have been more and more isolated by separation. 

 A few of the shells have spread far downstream, even to the Muscle Shoals in northern Alabama, 

 but in general with the perfecting of the drainage the habitat of lo in the Tennessee tends to 

 become extinct. To-day below the mouth of the Clinch in the Tennessee these shells do not 

 appear to flourish as they do above this point. 



The general relations of these shells to one another and their hypothetical migrations are 

 indicated in the following diagram. (Fig. 1, text figure map.) This should be compared with 

 plate 1, on which is shown a diagram of the present distribution of the different forms. 



To understand the evolution of the spinose shells from the smooth or undulate it is desirable 

 to form some idea of the relations between the different characters which differentiate these 

 forms. If we look upon the smooth shells as those which lack the character of undulations 

 and of spines, those with undulations (as verrucosa) as having an additional character, and 

 spines as of still another character, we may gain some suggestions of value in attempting to 

 understand the relations of these shells to one another. The smooth shells live in such large 

 numbers as to make it clear that they must form at least one race which breeds true. Undula- 

 tions are also found among the relatively smooth Powell shells, but not to a marked degree, and 

 their presence suggests some degree of mixture of undulate individuals. However, undulations 

 are more conspiciously developed in the Upper Cliach, but to a less degree at the upper limit of 

 lo. In the North Fork of the Holston, undulations are well developed in some individuals at 

 Saltville, farther downstream, and in the South Fork the conditions are completely reversed. 

 Here the undulate shells make up the mass of the population and appear to be a race which 

 breeds true, and the smooth shells are the exception. That undulations are thus able to form 

 a local race suggests that we are dealing with a character of some individuality, whose presence 

 in the smooth shells may be looked upon as so much contamination from another strain with 

 which they are fertile when crossed. Undulations progressively decline with departure from 

 the South Fork of the Holston. The relations of these forms seem relatively simple. On the 

 other hand, spinosity presents a much more complex problem, because it not only varies in 

 degree, but also in the time of its appearance in the life of the animal. It may develop only late 

 in life, or it may do so from its earHest postembryonic development; and at least several of these 

 different degrees of spinosity are represented by local races. Some of the degrees of spinosity 

 are dependent on the size of the stream, because in general large shells live in large streams, 

 and large shells have large spines; but the time of life at which spinosity develops appears to be 

 more of a racial character. For this reason the time of life at which spines develop will be given 



