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 191 7, 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, 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 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 phylogeny of the group, and evidence adduced to indicate 
that it was a depressed, flattened, free-swimming animal of few segments. 
