5 ° PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
tological characters, their geological age, correlations, physical features, 
and conditions under which the respective beds were deposited, as far as 
the material permits any conclusions in these respects. In this general 
part, several stratigraphical observations of Mr. Hatcher have been antici- 
pated, which properly belong in the geological part of this work. But 
since the stratigraphical evidence is absolutely necessary for the correct 
understanding of the palaeontological facts, Mr. Hatcher has kindly fur- 
nished all the pertinent data, and permitted their use by the writer. It is 
hardly necessary to say that the credit for all stratigraphical observations 
is to be given to Mr. Hatcher, and the writer is satisfied that they are 
entirely trustworthy, since they agree admirably with the palaeontological 
facts, and sufficiently explain them. 
In regard to the identification of species, I am greatly indebted to Dr. 
H. von Ihering of Sao Paulo, Brazil. He not only gave his opinion in 
doubtful cases submitted to him, but sent to the Princeton Museum a 
collection of Patagonian as well as Entrerios fossils, which aided me 
materially in the correct identification of our material. It is only proper 
to express also in this place my thanks for this valuable assistance. 
In the systematic arrangement, I shall follow Zittel’s Palaeontology 
(1880 and 1885), but for the Mollusca I shall use the English edition of 
this work (Dali and Pilsbry, 1900). I regret very much that the latter 
did not come into my hands before the figures for the plates were drawn 
and sent to the printer, and this explains the fact that the arrangement 
of the species on the plates is different from that in the text. But I think 
this will prove to be only a slight inconvenience, and will certainly be 
counterbalanced by the improvement in the systematic part of the text. 
