14 EDITOR’S PREFACE. ~ 
It was Professor Hyatt’s custom, in citing the authority for a species, 
to give the name of the author who first referred the species to the genus 
in which it is now retained, instead of citing the one who first proposed the 
specific name. Thus he wrote Placenticeras placenta Meek, although the 
form was described as Ammonites placenta by DelKay. This is not in 
accordance with the rule usually followed by zoologists, and the citations 
have been changed so that each specific name is followed by the name of 
the original describer of the species, with the author’s name in parentheses 
if the generic reference has been changed. 
When the figured specimens were at the Geological Survey in the 
hands of the artists, Professor Hyatt requested the writer to examine them 
carefully and give him notes and comments as to the localities and horizons 
of the specimens from his own collection and from other museums. The 
quoted statements concerning various species that appear in the manuscript 
were extracted from the notes the writer then forwarded to him. 
The appended tabular statement is intended to show the relative 
positions of the various marine Cretaceous horizons and formations men- 
tioned in the text. It does not imply accurate correlation of American and 
European horizons, the line between Lower and Upper Cretaceous being 
probably somewhat higher in America than in Europe. 
The figured specimens that are mentioned in this paper as belonging 
to Professor Hyatt’s private collection have now become the property of 
the Museum of Comparative Zoology in Cambridge. 
The writer has supplied the table of contents, the formal bibliographic 
list, and the descriptions of the plates after Pl. XIX. 
T. W. Sranron. 
