I wish to express my gratitude to a great number of persons who over the years have contributed encouragement, inspiration, and practical help. I am particularly indebted to the late Dr. Eugene Richardson and to Matthew Nitecki of the Field Museum of Natural History, Chicago, for making the museum collections available for the selection of specimens to be used in this work, for the loan of those specimens, and for many helpful discussions.

A great debt of gratitude is due to Dr. Jan Bergstrom who has supported the development of this work through the years with encouragement and inspiration. While I was struggling with the first edition of this book, Dr. Bergstrom, then at the University of Lund, helped me with much-needed information regarding the classification and life habits of trilobites. We then collaborated in fieldwork in Newfoundland and in the writing of a manuscript about the Paradoxides fauna mentioned in Appendix A of my first edition of this book. It has been a treasured experience to unscramble together the many puzzles presented by our findings. A field trip through classical Swedish trilobite localities, guided by Dr. Bergstrom, has been the highlight of a trip to Sweden some ten years ago. As this second edition was taking shape in recent years, Dr. Bergstrom, now at the Swedish Museum of Natural History in Stockholm, did help me once more with much dedication. My views concerning trilo-bites were updated and corrected through a critical reading of the manuscript and much appreciated advice. During a recent visit to Stockholm, Dr. Bergstrom gave me the opportunity to peruse the trilobite collections and to photograph several type specimens of historic value at the Swedish Museum and at the Geological Survey of Sweden and the Paleontological Institute of the University in Uppsala.

The section on trilobites eyes would not have existed without the contributions by Dr. E. N. K. Clarkson of the Grant Institute of Geology in Edinburgh. For his magnificent photographs and the communication of many of his results, a great deal of appreciation is due. It has been most rewarding for me to collaborate with Dr. Clarkson on the writing of a manuscript on the optics of the phacopid eye lenses. This edition contains the conclusion of this investigation, which was still in progress at the time the first edition of the atlas was published. My visit to the Grant Institute of Geology in Edinburgh, and memorable field trips to the Pentland Hills and Girvan guided by Dr. Clarkson, have been a much appreciated opportunity.

Through the courtesy of Dr. J. Cisne, I had access to the collection of Beecher's Utica shale trilobites as described in section 3.2. In addition to the loan of many of the trilobites, many X-ray negatives were most generously contributed by Dr. Cisne, to whom I am deeply grateful.

I wish to thank Dr. Bernhard Kummel of Harvard's Museum of Comparative Zoology for allowing my perusal of the museum collection and for the contribution of some of the most beautiful trilobites in the entire atlas, from Bohemia in particular. I am also grateful to Dr. N. Eldredge of the American Museum of Natural History and to Dr. K. Towe of the Smithsonian Institution for their discussions and useful communications of their work. Dr. A. M. Ziegler at the University of Chicago has helped on many occasions with his stimulating criticism.

Further appreciation is due the superintendents of many quarries for their permission to collect fossils; particular thanks go to Mr. H. Nester of the Consumer's Company Quarry at McCook, Illinois; Mr. J. Riordan of the Lehigh Stone Company, Lehigh, Illinois; and the Medusa Cement Company at Sylvania, Ohio.

Many more persons have helped-with permission to collect, to photograph specimens from their collections, with loans of specimens, and with precious gifts.

In preparing this second edition, I was privileged to have access to the extraordinary trilobite collection of David C. Rilling, M.D., of Sellersville,

Pennsylvania. Many exceptional specimens were photographed on location with his kind assistance. Another mutual friend, Pio Pezzi, M.D., of Huntington Valley, Pennsylvania, has loaned several valued specimens. A word of appreciation is also due a number of fossil dealers and trilobite preparators, from whom many specimens have been obtained or loaned.

Among these, Mr. Afton Fawcett of Hurricane, Utah, has provided many trilobites, collected and prepared with professional care. Robert Carroll of Ann Arbor, Michigan, did contribute with the superb preparation of several trilobites exhibiting extremely delicate structures. To be mentioned also are Dave Douglass of Evanston, Illinois; Michael Thomas of York, Pennsylvania; Leon Theisen of Hill City, South Dakota; Eddie Cole of Delta, Utah; Bruno Corti of Milan, Italy; and Franco Todde of Iglesias, Sardinia, Italy, for enabling me to portray specimens from their collections.

I am particularly indebted to my sons Emile and Matteo, who since they were first able to climb boulders have very successfully competed with their father in finding countless trilobites.

Once, many years ago, in Rome, I had the opportunity of meeting

Franco Rasetti. "Keep them sharp," he kept telling me, referring to the tools needed to free trilobites from their matrix. The memory of that meeting, and of much constructive advice, is still cherished. When presented with the first edition of my atlas, he admitted surprise at finding out that so much could be said about Post-Cambrian trilobites. Those who know of Rasetti's contributions to physics and paleontology will understand the reasons for a particular devotion to this great scientist on the part of another trilobite-loving physicist.

Trilobites have pervaded much of my personal life, vacation time, as well as my home environment. The second edition of my book would not have materialized without the devoted help and encouragement of my wife, Nika, who willingly endured the hardships of many field trips and countless hours of digging. For her patience and endurance, in the field and at home, I am deeply indebted.

It is a pleasure to acknowledge the help of several students, apprentices in the art of photographic printing. To be mentioned are Margarita Garcia, Marian Harris, Claudine Malik, and Tara Shochet.

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