nicki.es ANn n.vssi.ER.] EXPLANATORY NOTES. 11 



This list will give ii very fair idea of what has thus far been done for 

 the br3'ozoa. For kindly help which has done much to make this list 

 as full as it is, we haye to thank Dr. Josua Lindahl, the accomplished 

 Director of the Cincinnati Society of Natural History, and manj'^ w'ork- 

 ers abroad, more particularly Prof. Sidney F. Ilarmer, Mr. Arthur 

 William Waters, Dr. Ed. Pergens, Prof. Hinrich Nitsche, Prof. E. 

 Ehlers, Prof. F. A. Smitt, M. Gustaye Dollfus, Inspector G. M. 11. 

 Leyinsen, Mr. Robert Etheridge, jr., and Dr. A. Neyiani. 



It remains to acknowledge our very great indebtedness to Mr. E. O. 

 Ulrich, of whose lifelong studies of the bryozoa, published and unpub- 

 lished, carried on with a most noble integrity of purpose, often under 

 most untoward circumstances, we have been freely permitted to avail 

 ourselves. 



To the Director of the United States Geological Survey, Hon. 

 Charles D. Walcott, whose keen appreciation of all that tends to fur- 

 ther geological and paleontological science has been many times proved, 

 we are indebted for many favoi-s. 



EXPI.AXATORY NOTES. 



It has seemed advisable to give a few brief explanatory notes. 



1. In the catalogue of genera and species, valid genera and species 

 are in l)lack-faced type; an interrogation point following the generic 

 name indicates that it is not certain that the species belongs to the 

 genus under which it is placed; two interrogation points following 

 the generic name indicate that, while we have left the species where it 

 was referred b}' the author, we know that it does not belong there, 

 but we do not know where it does belong. As a rule, we have excluded 

 these species from the tables giving the numbers of species. 



2. In giving the geologic formation we have usually adhered to 

 connnonly accepted designations, though this sometimes produces 

 seeming ambiguity. Thus, when Trenton appears alone, followed by 

 the locality, it denotes the group; when followed by a parenthesis 

 including Black River or Stones River it denotes the period. Trenton 

 in parentheses means the group. For the Trenton period Clarke and 

 Schuchert' have proposed the name Mohawkian. 



After the term Cincinnati (period) we have given the group in 

 parentheses. This we have done because verj^ few authors have 

 given the group from which their species were derived. The recog- 

 nition of these groups is comparatively recent. 



The Lower Helderberg we have included with the Silurian in our 

 table of formations. It is still an open question to which age it more 

 properly belongs, the Devonian or the Silurian. 



We have continued to use the name Upper Helder])erg, though the 

 name Ulsterian, lately proposed by Clarke and Schuchert,^ may prove 



' Science, new ser., X, 1S99, pp. S74-S78. 



