No. 2.1 SNAILS OF THE GENUS 10— ADAMS. 79 



into turrita. The highest upstream that turrita was found aHve in the French Broad was at 

 Seven Island Shoals (lot 156, No. 55). In the same vicinity, lot 195, I found a good series of 

 dead shells, the small individuals of which can not be distinguished, by inspection, from noli- 

 chuckyensis, but the largest individuals are much larger and heavier than the largest shells from 

 the NoHchucky, as was to be expected from the larger size of the stream. These large dead shells 

 resemble so much the very large turrita found much farther downstream (lot 187, Belief onte, 

 Ala.) that had not the living shell been found in the same vicinity I would have considered 

 it most likely that these shells had been transported by man. It is very surprising that the 

 turrita shells just mentioned are the only ones found in the French Broad. In the Tennessee 

 proper, however, turrita is a relatively abundant shell, as is shown by its presence in large num- 

 bers in lots 100, 101, 102, and 103, from near Knoxville. In these lots turrita grades into the 

 form angitremoides and into spinosa and some individuals are practically indistinguishable from 

 nolichuckyensis. Of course these lots are composed of relatively immature shells, but to a cor- 

 responding degree they are well preserved and show that the apices are spinose. Turrita is 

 abundant on the Little River Shoals, at the mouth of Little River. Below Little River the 

 largest number of live turrita came from (lot 130) Loudon, and with farther advance downstream 

 these shells become very large. Turrita extends downstream the farthest of any member of the 

 genus and finds its extreme limit on the Muscel Shoals of Alabama, and thus it has the most 

 extensive range of any form in the genus. 



Angitremoides. This rather anomalous shell is recognized only in the lower French Broad 

 where it occurs in lots 136, from Dandridge, lot 137, and from Hanging Rock Shoals; and in the 

 upper Tennessee, lot 124, from Loneys Island, and in lot 152, from Loudon. The largest 

 series of this kind found in any single locaHty came in lot 124, in which 35 out of 36 shells were 

 of this form. Were it not for such a series this form might be considered hs composed only of 

 extreme individual variations. It is significant that angitremoides is most abundant ia the 

 same vicinity from which lots 100, 101, 102, and 103 came, as these are the series of turrita 

 shells, to which they are most closely related. This form is clearly related to the NoHchucky 

 shells and to turrita. 



The general geographic relations of these shells is summarized on plate 1. The approximate 

 range of each form is shown on the upper part of the plate, and on the lower part is given a figure 



of the kinds of shells, as represented by the type specimen or representative specimens. 



» 



7. THE MIGRATIONS OF THE ENVIRONMENT AND THE MIGRATIONS AND RELATIONS OF lO. 



In the earlier chapter on the evolution of the gross environment the main facts in the his- 

 tory of the physical environment were outlined, but the relation of the changes to lo were not 

 elaborated. I wish now to summarize these facts and inferences and indicate their relation 

 to the migrations, history, and interrelations of these sheUs. Such a discussion must be pro- 

 visional and wiU require revision with advances in our knowledge of the interpretation of the 

 environment and with similar advances in our interpretation of the snails, because relative 

 values are constantly changing. This discussion is built upon several assumptions, and first 

 of these is that the responses of the animals to the present conditions give us the most impor- 

 tant clues to their responses in the past. To-day these shells inhabit smaU rivers, rapidly flowing 

 well oxygenated waters on shoals, are larger in larger streams, show greater spinosity down- 

 stream, inhabit streams draining lime-bearing rocks and show local differentiation. It is not 

 necessary to assume that the original form of shell was invariable because it may have been 

 smooth and undulate, as the Saltville shells of to-day. I infer that incipient spines, apparently 

 a different character than undulations, developed in the downstream portions of a transversely 

 flowing stream, and have migrated upstream and to the northeast. Spinosity apparently 

 developed in the vicinity of the western part of the South Fork of Fluvialis River, or in Lou- 

 donensis River. With the development of longitudinal streams there has been a migration 

 of the streams to the northeast, and the smooth shells of the present headwaters probably origi- 

 nated farther to the south. While the habitat of lo has migrated upstream it has at the same 

 time tended to become extinct farther downstream, primarily because the various base-levels 



17829°— 15 6 



