xii PREFACE. 



puMished papers relating to tlie 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 Orthidje, the 

 strophemenoid and streptorhynchoid forms in their varied aspect and modifi- 

 cation, and through the leptaenoid forms to Chonetes and the Productidm 

 proper, with which the series seems naturally to end. 



All tlie spire-bearing forms, all the Rhynchoneludm and Pentamerwm 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 the}^ 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- 

 si, \ 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 



