6 DlClYOBl'ONdlDyJi. 



Through the kindness of Prof. I. C. Russell, of the University of Michigan, 

 was ol)taine(l the use of the original specimen of Lyrodictya Romingeri and a 

 few othei's belonging to the museum of that institution. 



From the American Museum of Natural History, through Prof. 11. P. 

 Wun'FiELD, was received on loan all the material representing the Dictyo- 

 spoNGiD^ in the Hall collection together with that utilized bj' this author 

 as the basis of his published determinations. 



Through Prof. J. F. Kemp, of Columbia University, the specimens from 

 the loAver Carboniferous rocks t)f Ohio, collected by the late Dr. J. S. 

 Newberry and belonging to the museum of that institution, were kindly 

 loaned. 



Interesting specimens from the Chemung and Waverly groups, which 

 have been utilized in our illustrations, -were obtained from the Peabody 

 Museum of Yale University through the favor of Prof. C. E. Beecher. 



Some important specimens from the Utiea slate, fully discussed herein, 

 have been received from the National Museum at Washington, by the courtesy 

 of Mr. Charles Schuciiebt and the late Dr. G. Brown^ Goode. 



Prof. Albert A. Wright, of Oberlin College, generously loaned speci- 

 mens from the collections of that institution, among them a very large and 

 interesting example of the I'are genus Hypiiant^nia. 



Through the favor of Prof. E. W. Claypole, of Akron, Ohio, ^vas obtained 

 the original specimen of Oleodictija Olaypolei. 



The Rev. J. H. McKee, of Olean, has permitted the examination and use of 

 Chemung sponges in his own collection and that of the Olean Society of 

 Natural Science. 



The delineation of the accompanying figures has been largely done by 

 Mr. George B. Simpson, who has produced them in a highly skillful and artistic 

 manner. Some of the earlier figures were made by Mr. Ebenezer Emmons. 

 The illustrations of spicules are from camera drawings by Mr. Clarke. 



The lithographic work has been admirably executed by Mr. Philip Ast. 



In the correction of the proof sheets much assistance has been rendered 

 by Mr. Jacob Van Deloo. 



To the persons mentioned and to all others who have been of assistance 

 to the work, I offer the assurance of my obligation and consideration. 



JAMES HALL. 



