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 accom- 

 pany 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 com- 

 pleted in 1917, it contained an account of the appendages only. Thoughts upon the prob- 

 able 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 Univer- 

 sity 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, Calymcnc, 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 appen- 

 dages of Neolenus, Isotelus, Ptychoparia, Kootcnia, 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 phytogeny of the group, and evidence adduced to indicate 

 that it was a depressed, flattened, free-swimming animal of few segments. 



