No. 2.] SNAILS OF THE GENUS 10— ADAMS. 73 



upper and more rapid parts of the stream the shells are thicker and smooth; farther down they 

 are thinner and smooth; and still farther downstream they have well-developed spines and a 

 thin shell. This longitudinal replacement of forms is a condition frequently observed in streams. 



Voigt (1904, 1907) has shown that some planarian worms in the mountain streams of 

 Germany are found in an orderly arrangement. In the extreme headwaters of certain brooks 

 Planaria dlpina is found, farther downstream pioneers of Poly cells cornuta are found with it, 

 stiU farther down P. cornuta is alone. Still farther down pioneers of Planaria gonocepTiala are 

 associated with cornuta, and still farther downstream gonocepJiala occxu's alone. A similar con- 

 dition is found among the crawfishes, as has been shown by Williamson (1901, p. 11), who says: 

 "A collector starting at the headwaters of Squaw River (in western Pennsylvania) would find 

 hartonii; following the stream he would soon notice rohustus among his captures; then an occasional 

 propinquus { = ohscurus) , till finally hartonii would become rare and disappear, then robustus 

 would disappear, and near the mouth of the creek he would find only the species propinquus 

 { = ohscurus) ." 



Nor is this condition limited to invertebrates because it was long ago pointed out by Agassiz 

 in 1850 and 1854, as applyiug to stream fishes, and has since been studied by Cope in 1868 and 

 Jordan in 1878, in the southern Appalachians; and very recently in small streams by Shelf ord 

 in 1911. 



In view of these facts, how do the various kinds of animals reach their respective segments 

 in a stream where they find favorable conditions of life ? The answer to this question must vary 

 with the history and ecology of the animals considered. There are evidently many causes which 

 produce this result. Transportation will apply to some animals, others have reached these 

 conditions in the past, under other climatic conditions (Voigt), as in the case of the planarians 

 in the alpine streams. Some, as in the case of animals possessing good powers of locomotion, 

 by their own movements alone, and still others by changes in the topography and drainage (Cf . 

 Adams, 1901). In the case of lo it seems probable that the main factors which have influenced 

 the longitudinal arrangement have been the movements of the animals themselves, in combi- 

 nation with the migration of the habitat, as the shoals have been changed with crustal movements 

 and the processes of baseleveling. What are considered to be the most ancient forms, the smooth 

 shells, are in the headwaters, in those drainage lines which have encroached upon the very 

 ancient New River divide. This view favors the idea that in normal stream development the 

 older relatively sedentary forms will occur upstream the farthest, because they were the first 

 invaders and have migrated with the stream, and that later the stream was successively invaded 

 by the forms which differentiated later. The most spinose shells occur downstream farthest 

 from the headwaters, and they show in their early ontogenetic development of spines that they 

 are more highly specialized. 



It thus appears highly probable that the order of arrangement in some ancient streams 

 of normal development is indicative to some degree of the antiquity and degree of specialization 

 of the group. If the stream has had a very complex history, with considerable deviations from 

 the normal or ideal development, this arrangement may be greatly obscured. It will be best 

 shown in those parts of the stream which have the most normal development. In the Ten- 

 nessee system I believe this applies best to their upper courses, and it is in these that we find the 

 shehs showing parallel lines of development, as in the Powell and Clinch, but in the upper Hoiston 

 system, the complex piracy in its upper course appears to have caused the peculiar inverted 

 arrangement with some smooth shells far from the headwaters. It is possible that where these 

 smooth shells now live (near RogersvUle) was once the headwaters of a northward flowing 

 stream, which was captured, its current reversed, and the snails thus given their anomalous 

 location. In my field work I had anticipated that ancient types of shehs might be found in the 

 extreme headwaters, and for this reason a special effort was made to determine the kind of shell 

 and the upper limit of these shells in each stream, and this idea may prove useful to others in 

 similar studies. 



