300 PALiEONTOLOGY 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 Mprchison 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 

 we 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 Eurypterus and the associated crustaceans in New- York, are 

 interesting and important, both in regard to the horizon Avhich 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, palaeozoic 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 palajontologist have induced him 



