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IV I am also indebted for information and the loan of precious typo material to Dr. Paul Bartsch of the National Museum, Mr. William J. Clench of the Museum of Comparative Zoology, Dr. G. Dallas Hanna and Leo G. Hertlein of the California Academy of Sciences, Dr. Frank C. Baker, University of Illinois, Dr. Stanley T. Brooks, Carnegie Museum, and Mr. Calvin Goodrich, Museum of Zoology, University of Michigan. Dr. Bernard Rensch lent type material from the Zoologisches Museum in Berlin, and Lieut.-Col. A. J. Peile compared various specimens with types in the British Museum. The book is much the better for this friendly cooperation.

In the Academy the long task of preparing the work has been constantly encouraged by the President, Mr. C. M. B. Cadwalader, whose interest made the publication possible. Mr. J. A. G. Rehn, Chairman of the Publication Committee, gave liberally of his time and expert knowledge of engraving and printing. Mr. V. S. L. Pate assisted in preparing the manuscript for the press. In the proofreading I owe much to Mr. C. Bernard Peterson's accurate eye. He also prepared the Index.

Finally, the illustrations of shells, which, in a descriptive work on mollusks are considered as essential as the text, have mostly been prepared by Miss Helen Winchester, whose collaboration has been indispensable to the usefulness and the appearance of the work.