General Remarks Concerning Fossilized Arachnida ACKNOWLEDGEMENTS It gives me great pleasure to acknowledge my indebtedness and to express my sincere thanks to the following institutions and individuals: The Authorities of the British Museum of Natural History and especially Mr. W. N. Edwards, Keeper of Geology, for the loan of their collection of British Palaeozoic Arachnida. The U. S. National Museum and especially Drs. Alexander Wetmore, Director, R. S. Bassler, Head Curator of the Department of Geology, and G. Arthur Cooper, Curator of Invertebrate Paleontology and Paleobotany, for the loan of several types of American Palaeozoic Arachnida. The American Museum of Natural History and especially Dr. Otto Haas, Curator of Invertebrate Paleontology, for the loan of the type of Proscorpius osborni. The Peabody Museum of Yale University and especially Dr. Carl 0. Dunbar, Director, for the loan of their types of Palaeozoic Arachnida. Dr. John S. Nicholas, Director of the Osborn Zoological Laboratory, for the extension of working facilities of the laboratory and especially of the photographic equipment. Dr. Grace E. Pickford, Research Associate of the Bingham Oceanographic Laboratory, for sustained interest in my work and the tedious labor of reading and correcting my manuscript. Dr. Dorothea Rudnick, Secretary of the Connecticut Academy, for valuable suggestions and help in reading the proof. A STUDY OF PALAEOZOIC ARACHNIDA INTRODUCTION The great advance in our knowledge of the comparative internal anatomy, development and neurology of Recent Arachnida and of other related groups of Arthropoda during the past fifty years opened new avenues of approach to their study by making correlation of their external features with the rest of their organization not only possible, but imperative. No more can mere reshuffling of taxonomic units on the basis of external characters alone be accepted as an adequate reason for a new "natural" classification, unless other important characters involving fundamental anatomical and embryological features fall in line and furnish the required corroborative evidence. External resemblances, when unsupported by such evidence, are often quite deceptive. This is particularly true in the case of such arthropods as Crustacea, but applies to Arachnida as well. On the other hand, great external differences may be exhibited by closely related groups. To be sure, identification of species, genera and families, and to a certain extent of even higher taxonomic units of arthropods has in the past always depended upon externally visible structures and will continue to depend upon them in the future, unless one is prepared to sacrifice the specimens for a study of their internal anatomy. But the choice of such characters and their use must be regarded merely in the light of their utility, not in themselves indicative of fundamental differences. One should not belittle their practical value, but at the same time should not exaggerate their phylogenetic importance. The student of fossil Arachnida has no other alternative than the use of external characters, because internal organs are scarcely ever preserved and then only very imperfectly. When it becomes necessary to correlate the external features of fossils belonging to extinct orders, this can be done only by reference to similar structures found in now living Arachnida of other orders. In other words, in case of fossils we have to depend upon analogies. This in itself requires great circumspection. But additional, often serious difficulties arise owing to imperfect preservation, distortion, superimpression and scarcity of material. As we shall see further, misinterpretation of visible structures happened repeatedly even on the part of distinguished investiga 75 |