X PREFACE. 



The collections for the illustrations of this volume have been chiefly made 

 by myself and by assistants formerly occupied in collecting material for the 

 Palaeontology of the Stivte, and later by assistants connected with the State 

 Museum. Collections have been purchased by the author from Mr. Edmond 

 De Cew, and of the late Mr. John De Cew, of Cayuga, Ontario, Canada; the 

 latter having also matle special collections for use in this work from the siliceous 

 layers of that ptixt of the Helderberg group in Canada. This worii is indebted 

 to Mr. Victor W. Lyon, of Jefferson, Indiana, for important collections made 

 at the Falls of the Ohio. These specimens were in a perfectly preserved con- 

 dition, and have constituted an important element for the illustration of this 

 volume. 



The original drawings have all been made by Mr. George B. Simpson, who 

 has critically studied all the species, and, with the appreciation of a naturalist, 

 has faithfully represented the delicate structure of these organisms in the 

 illustrations accompanying this volume. Credit is due him for the preparation 

 of a large part of the specific descriptions. In the execution of the drawings 

 Mr. Simpson has shown the same artistic ability which has characterized his 

 previous work. 



The lithography has been chiefly done by Mr. Philip Ast, of whose ability 

 in reproducing the original drawings upon stone, we cannot speak too highly. A 

 few plates by the late Mr. H. Bergmann, give very accurate and elaborate repre- 

 sentation of Favosites Helderbergia and of species of Fistulipora, Ptilodictva, 

 etc., of the Lower Helderberg group. More recently some plates of the Upper 

 Helderberg and Hamilton groups have been done by Mr. Paul Riemann. 



We are indebted to Mr. Charles E. Beecher for the preparation of the Synopsis 

 of tiie Genera and Sub-genera described and illustrated in this volume ; and also 

 for the arrangement of the explanations of plates, with references to the pages 

 of the volume and the localities of the species. 



Mr. Beecher and Mr. Simp.son have rendered assistance in proof-reading while 



the volume has been passing through the press. 



THE AUTHOR. 

 Albamt, August, 1887. 



