methods unknown, their results were not accepted; they were regarded as startling, as iconoclastic, and even unreliable." He therefore decided to rework his material and to illustrate his publications with enlarged photographs. The specimens were black, there was little relief between fossil and matrix, and the ammonium chloride process of coating them white and photographing under artificial light was unsuitable. Nevertheless, after many trials, he finally succeeded in making fine enlarged photographs of the trilobites immersed in liquid Canada balsam, with a contact cover of glass through which the picture was taken, the camera standing vertically over the horizontal specimen. Beecher had completed this work in 1903 and in the winter of 1903-1904 was making the drawings, nearly all of which are here reproduced. On Sunday morning, February 14, 1904, as he was working at home on a large wash drawing of Cryptolithus, death came to him suddenly, leaving the trilobite problem but partially solved.

When the writer, in the autumn of 1904, succeeded Professor Beecher in the chair of Palæontology at Yale, he expected to find considerable manuscript relating to the ventral anatomy of the trilobites, but there was only one page. It was Beecher's method first to prepare and thoroughly study the material in hand, then to make the necessary illustrations, and between times to read what others had written. There was no written output until everything had been investigated and read, certain passages being marked for later reference. Then when all was assimilated, he would write the headings of topics as they came to him, later cutting them apart and arranging them in a logical sequence. When the writer visited him in his home in January 1904, he was primed for his final trilobite memoir, but the writing of it had not been begun.

The writer has never made the trilobites his special subjects for study as he has the brachiopods, and therefore felt that he should not try to bring to light merely the material things that Beecher had so well wrought out. It seemed at first an impossible task to find the specialist and friend to do Beecher justice, but as the years have passed, one of Beecher's students, always especially interested in trilobites, has grown into a full appreciation of their structures and significance, and to him has fallen the continuation of his master's work. If in the following pages he departs here and there from the accepted interpretation and the results of others, it is because his scientific training, in desiring to see with his own eyes the structures as they are, has led him to accept only those interpretations that are based on tangible evidence as he understands such. Furthermore, in seeking the relationship of the trilobites to the rest of the Arthropoda, his wide study of material and literature, checked up by the ontogeny of fossil and recent forms, has led him in places from the beaten path of supposedly ascertained phylogenies. His results, however, have been won through a detailed study of the interrelations of the Arthropoda, starting from the fact that the Trilobita are chronogenetically the oldest and most primitive. The trilobites are held by him to be the most simple, generalized, ancient Crustacea known, and the progenitors, directly and indirectly, of all Arthropoda.

It is now twenty-six years since Professor Beecher began his publications on the class Trilobita, and in commemoration of him and his work, Professor Percy E. Raymond of Harvard University presents this memoir, to bring to fruition the studies and teachings of his honored guide. It has been with Professor Raymond a labor of love, and it is for the writer of this foreword a long-desired memorial to the man to whose position in the Museum and University he had the privilege of succeeding.

Yale University, New Haven, Connecticut.

PREFACE.

The primary object of this memoir is, as has been stated by Professor Schuchert, to. rescue from oblivion the results of the last few years of Professor Beecher's investigations on the ventral anatomy of trilobites. Since he left his data in the form of drawings and photographs, without even rough notes, it became necessary, in order to write a text to accompany the plates, to restudy the entire subject. Under these circumstances, it seemed best to include all that is known about the appendages of trilobites, thus bringing together a summary of present information on the subject.

The growth of the memoir to its present size has been a gradual one. As first completed in 1917, it contained an account of the appendages only. Thoughts upon the probable use of the appendages led to the discussion of possible habits, and that in turn to a consideration of all that is known or could be inferred of the structure and anatomy of the trilobite. Then followed an inquiry into the relationships to other Arthropoda, which ultimately upset firmly established preconceptions of the isolated position of the group, and led to a modification of Bernard's view of its ancestry.

During the progress of the work, I have had the opportunity of examining most of the known specimens retaining appendages. From the Marsh collection in the Yale University Museum were selected the forty-six specimens showing best the appendages of Triarthrus, Cryptolithus, and Acidaspis. Dr. Charles D. Walcott very kindly returned to the Museum of Comparative Zoology the slices of Ceraurus, Calymene, and Isotelus which were the basis of his paper of 1881, and which had been loaned him for further study. He loaned also eight of the more important specimens of Neolenus serratus, and two of Triarthrus becki. At the United States National Museum I saw the specimens of Isotelus described by Mickleborough and the isolated limbs of Calymene from near Cincinnati. The Isotelus at Ottawa I had already studied with some care while an officer of the Geological Survey of Canada.

This memoir consists, as shown in the table of contents, of four parts. The appendages of Neolenus, Isotelus, Ptychoparia, Kootenia, Ceraurus, Calymene, and Acidaspis are discussed, as fully as circumstances warrant, in the first part, and new restorations of the ventral surfaces of Neolenus, Isotelus, Triarthrus, Ceraurus and Cryptolithus are included It is not supposed that these restorations will be of permanent value in all of their detail, but they are put forward as the best approximations to the real structure that the writer is able to present from the materials so far discovered. I am greatly indebted to Doctor Elvira Wood for the care and skill with which she has worked up these restorations from my rather sketchy suggestions. She has put into them not only a great amount of patient work, but also the results of considerable study of the specimens.

Part II is a discussion of the internal anatomy of the trilobite and a brief statement of some of the possible habits and methods of life of these animals. Part III, which begins with a survey of the relationships of the trilobites to other Arthropoda, is largely taken up with an attempt to demonstrate the primitive characteristics of the former, and their probable ancestral position. The form of the ancestor of the trilobite is deduced from a study of the morphology, ontogeny, and phylogeny of the group, and evidence adduced to indicate that it was a depressed, flattened,