INTRODUCTION 5 



the interior from youthful to old-age stages, which made it possible for us to understand the devel- 

 opment of the pseudospondylium. Doctor Chester A. Reeds of the American Museum of Natural 

 History sent us the Conrad types of Pionodevia, and through Doctor H. W. McGerrigle of Dart- 

 mouth College we were able to study a good series of Clarkella, Syntro-phina, and Finkelnburgia 

 from the Phillipsburg region of Quebec. Doctor E. M. Kindle of the Geological Survey of Canada 

 permitted the study of Billings' types of Orthis electra, now the types of our genus Archceorthis. 

 Professor Paul H. Dunn of Miami University helped us with material of Eridorthis. Through Pro- 

 fessor Percy E. Raymond, the Museum of Comparative Zoology of Harvard College loaned us a 

 collection of European brachiopods particularly rich in representatives from the vicinity of Lenin- 

 grad. Doctor B. B. Bancroft of the Sedgwick Museum, Cambridge, England, and Doctor A. Opilc 

 of the University of Tartu, Estonia, sent us very perfect and interesting specimens of their new 

 genera which are not well known in this country, and many of these specimens have been figured 

 by us. 



Through the courtesy of Doctors R. S. Bassler and C. E. Resser of the United States National 

 Museum the junior author was allowed to study the incomparable collection of Cambrian brachiopods 

 there preserved. Doctor E. O. Ulrich of the United States Geological Survey and the National 

 Museum loaned us some of his choice specimens of the very rare genera Mcewanella, Deltatreta, 

 and Finkelnburgia. Through Miss Winifred Goldring of the New York State Museum we had 

 access to specimens of the strange genus Australina, which we were able to determine definitely as a 

 representative of some division of brachiopods other than the Dalmanellidas where the senior author 

 had previously placed it. 



To Miss Clara Mae LeVene we are deeply grateful for her patient and exhaustive labors on 

 our manuscript, to Mr. Percy A. Morris for assistance in the preparation of the photographs and 

 figures, and to Doctor J. B. Knight, research associate in the Peabody Museum, for the generous loan 

 of instruments used in preparing fossils. 



Finally, special thanks are due our colleague. Professor Carl O. Dunbar, who has been our 

 patient advisor all through this work, and on whom the junior author has tried out most of our diffi- 

 culties relating to the interpretation of the structural details that we have observed. 



