CAMBRIAN GEOLOGY AND PALEONTOLOGY 



No. 4.— CLASSIFICATION AND TERAIINOLOGY OF 

 THE CAMBRIAN BRACHIOPODA^ 



By CHARLES D. WALCOTT 



(With Two Plates) 



CONTENTS 



Page 



Introduction i.^g 



Schematic diagram of evclution 139 



Development in Cambrian time 141 



Scheme of classification 141 



Structure of the shell 149 



Microscopic structure of the Cambrian Brachiopcda 150 



Terminology relating to the shell 153 



Definitions 154 



INTRODUCTION 



My study of the Cambrian Brachiopoda has advanced so far that 

 it is decided to pubHsh, in advance of the monograph,- a brief out- 

 Hne of the classification, accompanied by (a) a schematic diagram 

 of evolution and scheme of classification; (&) a note, with a diagram, 

 on the development in Cambrian time; (c) a note on the structural 

 characters of the shell, as this profoundly affects the classification; 

 and (d) a. section on the terminology used in the monograph. The 

 monograph, illustrated by 104 quarto plates and numerous text fig- 

 ures, should be ready for distribution in the year 1909. 



SCHEMATIC DIAGRAM OF EVOLUTION 



In order to formulate, as far as possible, in a graphic manner a 

 conception of the evolution and lines of descent of the Cambrian 

 Brachiopoda, a schematic diagram (see plate 11) has been prepared 

 for reference. It is necessarily tentative and incomplete, but it will 

 serve to point out my present conceptions of the lines of evolution of 

 the various genera, and it shows clearly the very rapid development 

 of the primitive Atrematous genera in early Cambrian time. 



1 Published by permission of the Director of the U. S. Geological Survey. 

 ^Monograph LI, U. S. Geological Survey. 



139 



