39 ® 
PALAEONTOLOGY OF NEW-YORK. 
mencement of vertebrate life is well and widely marked in the United 
States, showing the simultaneous appearance of this class of animals at 
numerous and distant points in the wide-spread ocean of that period. In 
the present state of our knowledge, we must regard the remarkable 
crustaceans marking the decline of the Silurian period as far less widely 
distributed than the fishes, which are considered as indicating the dawn 
of the Devonian period ; and relying upon these remains as characteristic, 
we find that the occurrence of these crustaceans is a far less persistent 
guide for determining the close of the Silurian period, than the presence 
of fishes for indicating the commencement of the Devonian. 
The generalizations offered by Sir Roderick Murchison are of great 
interest and importance, and (though in the United States the Oriskany 
sandstone may still be debatable ground) the views he has expressed re¬ 
garding the silurian age of these fossils are strongly sustained by all that 
w r e yet know concerning the relations of the strata in which they occur. 
These suggestions, joined with our knowledge of the age of the strata 
containing Euryptcrus and the associated crustaceans in New-York, are 
interesting and important, both in regard to the horizon which we are 
to rely upon as the recognized one between Silurian and Devonian strata 
in Europe, and as indicating the zone in which we are to look for these 
peculiar organisms. 
Throughout the many hundreds of miles of the outcrop of these forma¬ 
tions there is no mingling of the materials of the two epochs, and con¬ 
sequently no opportunity for the continuance of the fauna of the lower 
beds into the higher ones. We may except perhaps the Oriskany sandstone, 
where we detect a commingling of characteristic types of both lower and 
higher rocks; but even here the line is widely distant from the horizon 
of the peculiar crustaceans. With this strong physical demarcation, and 
the stronger, if possible, palieozoic distinction, we are scarcely prepared 
to conceive of these remarkable forms occurring in the beds of passage to 
the Old Red sandstone, and in the Devonian itself, as indicated by Mr. 
Salter in reference to Eurypterus acuminatus, E. megalops and E. symondsi. 
The evidences presented to this eminent palaeontologist have induced him 
