WATERLIME GROUP. 387 



Although found in a loose fragment of limestone, the character of the 

 rock and the associated fossils are conclusive in regard to its geological 

 position. 



These peculiar organisms, found thus between the Onondaga-salt group 

 below and the Lower Helderberg group above, I have referred rather to 

 the former ; for while the occurrence of the single carapace of a species 

 of Eurypterus, associated with known fossils of the Tentaculite limestone, 

 allies the formation in some measure with the rocks above, its lithological 

 associations are altogether with the rocks below. 



The sequence of strata above given shows conclusively that the Eury- 

 pterus beds are not to be regarded as of Devonian age. The rocks of the 

 Lower Helderberg are admitted to be Silurian ; and while no one would 

 include in the Devonian period rocks below the Oriskany sandstone, this, 

 from its mingling of species, may be regarded as debatable ground. The 

 Eurypterus (with a single exception) lies therefore between the Niagara 

 and Lower Helderberg groups, in strata above those containing the fossils 

 represented in the second volume of the Palaeontology of New- York, and 

 below those containing the fossils represented on the first seventy-nine 

 plates of the present volume*. 



It seems scarcely necessary, therefore, to discuss the question of the 

 age of the strata containing the Eurypterus ; and I introduce these re- 

 marks to correct the erroneous reference frequently made of these fossils 

 to the Devonian system. 



In comparing the rocks of this country with those of Europe, we learn 

 from the investigations of Sir Roderick Murciiison thq,t he hp,s always 

 found the strata characterized by these crustaceans immediately to under- 

 lie the rocks of undoubted Devonian age. This eminent geologist remarks t 



• The Eurypttrut should indeed more properly have formed the oonclnding part of tlie geoond volume 

 of the Pulieniitology of New-Vorki but at that time the ooUections possessed by me, or known to exist 

 in the State, were too meagre to aflurd means ft>r a proper illustration of the species, and much less for 

 adding to our knowledge of the structure of the animal. The descriptions were accordingly postponed to 

 the third volume, and are placed at the end of the Crustacea, not as indicating their geological relations, 

 but as a distinct group to be studied by themselves! while their geological place, in reference to the fossils 

 of this volume, would precede those of the first plates. 



