8 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xu. 



are at low water. In this way the territory from the Muscle Shoals in northern Alabama to the 

 upper limit of lo in each tributary of the Tennessee was sampled. Collectors were also secured 

 and instructed in the methods of collecting and preserving the shells, and thus valuable materials 

 were obtained. In the examination of the rivers all available methods were used, rowboats, 

 steamboats, journeys on foot, by horse, mule, and railways, and thus many hundreds of miles 

 of the rivers were traversed, and in all over 1,200 mUes of the river system was sampled, and 

 between 6,000 or 7,000 shells were secured with accurate data. Plans were made for breeding 

 experiments, but were necessarily abandoned after a start had been made for the lack of 

 adequate facilities. 



In attempting to inteipret the results of the present study the feeling has developed that 

 had some of the elaborate studies of heredity been built upon preliminary studies similar to 

 the present one the experimental results would have been of much wider application to the 

 conditions found in nature. The primroses Oenothera might be mentioned as such an example. 

 Certain groups of animals, particularly the song sparrows (Melospiza), the horned larks {Oto- 

 coris), and among the mammals the red squirrels (Sdurus), the white-footed mice (Peromyscus), 

 and our native rabbits (Sylvilagus, Lepus) are groups which would richly reward a student of 

 them, and also would do much to help give to modern taxonomy a somewhat different outlook. 



In general, in the earlier descriptive parts of the present paper no attempt has been made 

 to emphasize the interpretative aspect, because that phase is discussed more fully in later 

 chapters. 



When we consider the rapid rate at which our native plants and animals are being destroyed 

 by the encroachment of civilization, it will be realized that in a few generations a fairly full 

 account of many of our native species will be forever lost. I hope that the present record will be a 

 contribution to the preservation of such "vanishing data," and that the photographic record 

 and the collection wiU preserve a reliable sample of one of nature's vast experiments. 



THE HABITS AND LIFE HISTORY OF ID. 



HABITS. 



Very little detailed knowledge is recorded of the habits of lo. It is an aquatic, gUl-bearing, 

 operculate gasteropod which frequents only certain rivers of southwestern Virginia, eastern 

 Tennessee, and northern Alabama. Only a few references to its habits have been made in 

 the literature, and during my field work this phase of study was almost entirely neglected on 

 account of limited time at my disposal and the large area to be covered in the field work. The 

 best accounts of the habits are those by Lewis ('76) and by Wether by ('76). 



The most characteristic habit is that of frequenting the moderately rapid, probably 

 well oxygenated and shallow water of the shoals and rapids of rivers. In such situations the 

 water is generally only a few inches deep, as is usually the case in the smaller or headwater 

 streams, or even only a few feet deep in the large rivers. In general all the river tributaries 

 of the Tennessee inhabited by these shells may be considered as producing much of the shallow 

 rapid-water habitat; and it is on such shoals that these shells occur in the greatest abundance 

 and occupy the greatest area of habitat. The shells do not occur in creeks nor in rivers whose 

 drainage area is primarily from nonlime-bearing rocks. The deepest water in wliich these 

 shells occur in abundance is in the upper Tennessee from about KnoxvUle to Loudon, 

 Tenn. These conditions are also approximated in the lower part of the French Broad and 

 possibly in the lower Holston. As -a rule, the animals live in shallow water, but one shell 

 Got 156, No. 72) was taken by a collector in about 6 feet of water in the French Broad. 

 Below Loudon the abundance of live specimens decreases with suddenness. This apparently 

 . is due to several causes : The greater depth of the water on the shoals, the limited number of 

 shoals, their limited area, and possibly also to navigation improvements. Lot 151 from Day- 

 ton, Tenn., was taken in water which was generally from knee to waist deep upon the shoals. 

 Collecting is difficult under such circumstances, as the scattered individuals must be found 

 largely by feeling for them barefooted or with the hands. 



