Xll 
PREFACE. 
published papers relating to the Brachiopoda, during 1890 or 1891 which could 
not be cited in this volume, will here find the explanation. 
At the time this work was commenced the earliest known articulate 
Brachiopod had been described under the name of Orthis, and without having 
the knowledge or means to verify or disprove the character of this fossil, the 
genus Orthis was adopted for the basis of discussion. Had these older forms 
been better known, the order of the work might have been somewhat modified. 
The other associated and succeeding genera have been taken up and treated 
after the same idea as in Orthis ; limiting the discussion to those which seem 
to be a natural result of the modification of certain essential organic features 
characterizing the earliest forms of the orthoid type. 
Following this order and method we pass through all the Orthidn, the 
strophemenoid and streptorhynchoid forms in their varied aspect and modifi¬ 
cation, and through the leptsenoid forms to Chonetes and the Production 
proper, with which the series seems naturally to end. 
All the spire-bearing forms, all the Rhynchonellidn and Pentameridn as 
well as the terebratuloid forms have been left out of consideration in the present 
volume, believing that a more natural and useful classification will be found in 
the present adopted order and arrangement of the genera. Chapters upon 
the classification and broader relations of the genera are given at the 
conclusion of the two principal divisions of the work. The succeeding 
part ii of volume VIII will embrace the discussion of the genera under the 
several groups just mentioned, and they will be treated essentially in the same 
manner as in the present volume. The work on the second part is already far 
advanced; a large amount of material has been accumulated for study ; thirty- 
six plates have been lithographed, a considerable number of drawings have 
been made and a large amount of manuscript has been prepared. 
During the interval of more than twenty years from its commencement, 
great progress has been made in the study of both genera and species of the 
Brachiopoda. The late Thomas Davidson, LL. D., of Brighton, whose life had 
been devoted to the study of these organisms, living and extinct, made important 
contributions to our knowledge up to the time of his death in 1885. Essays 
