CONTENTS 



Page. 



I. Introduction 7 



II. Habits and life history of lo 8 



III. The races and forms of ib 10 



IV. The geographic relations of the shells examined 14 



1. Powell Kiver 15 



2. Clinch Kiver 16 



3. Holston River drainage system 18 



4. Nolichucky, Lower French Broad, and Tennessee Rivers 21 



V. Quantitative variation of Jo 27 



1. Variation in shell diameter 27 



2. Variation in globosity or shell index. 31 



3. Variation in spinosity 33 



VI. The evolution of the gross environment 40 



1. Introductory 40 



2. From the origin of the Appalachian land habitat to the formation of the Cretaceous Peneplain 44 



3. The Tertiary Peneplain and its drainage developments 46 



4. The Post-pUocene and recent cycles of drainage changes 49 



5. General summary 50 



VII. The geographic origin of the family 51 



VIII. Development of the shell and its sculpture 53 



1. Introductory 53 



2. The normal development of the shell and its sculpture 54 



3. Abbreviation and the inverse development of spinosity 60 



IX. General discussion and summary 65 



1. Inheritance in moUusks 65 



2. Ontogeny and phylogeny 68 



3. Agencies and means of dispersal 71 



4. Longitudinal distribution in streams 72 



5. Causes of variation 74 



6. Geographic range and variation of each form 76 



7. Migrations of the environment, and the migrations and interrelations of Jo 79 



X. Acknowledgements 85 



XI. Bibliography 87 



XII. Explanation of plates. (Plates 1-61) 94-184 



3 



