This study was made under the direction of Professor H. E. Vokes of The Johns Hopkins University, who proposed the problem and to whom the writer is indebted for constant advice, generous help and constructive criticism, and for making available to the writer his personal library. The writer is deeply grateful o t Dr. Julia A. Gardner of the United States Geological Survey for helpful advice and criticism and for her courtesy in permitting the use of some rare publications on the subject from her personal collection.
The writer wishes to express his profound gratitude and indebtedness to Professor J. T. Singewald, Jr., the Chairman of the Department of Geology, for offering the opportunity to pursue this study in his department.
To Dr. H. A. Rehder, Curator of the Division of Mollusks, United States National Museum, is due my most sincere appreciation for his personal guidance and advice on the problem and for offering the facilities of the Museum—without them this study would not have been possible.
The writer is also grateful to Dr. J. P. E. Morrison of the U. S. National Museum, Dr. R. Tucker Abbott of the Academy of Natural Sciences of Philadelphia, and to Dr. F. S. MacNeil, formerly of the U. S. Geological Survey, for the generous help and advice which they offered during the writer’s study at the Museum. He desires to express his gratitude also to the late Dr. J. B. Reeside, Jr., of the U. S. Geological Survey for permitting him