of America with those of Europe. 23 1 



of the Silurian and Devonian systems. In the second volume, 

 part I, of the Memoirs of the Geological Survey of Great Britain, 

 several species of what are termed Ortho7iota, but really, as we 

 think, Grammysia, are figured from the Silurian strata of Great 

 Britain, while others of Mytilns are very similar, if not identical, 

 with species from the Hamilton and Chemung groups of New 

 York. Thus far I have never seen, in the State of New York, 

 nor in any of the western states, a single species of bivalve of the 

 character of Grammysia, below the Gorniferous limestone, or even 



^^^^~ ^^B^b at — — — — — _ 



below the Hamilton group. These cingulated or obliquely fur- 



rowed and banded bivalves, are regarded as unerring guides to 

 the eye, of the formations ; and adopting the view of M. de Ver- 

 Benil, as to the horizon of the Devonian, they are all above the 

 Silurian system. There is still some mystery to us, on this side 

 of the Atlantic, how the Ludlow and Wenlock rocks should con- 

 tain fossils so very analogous to our Hamilton and Chemung fos- 

 sils, and still be of Silurian age, while many of the Devonian 

 Brachiopods of England are found mingled with these fossils in 

 the groups referred to here. If it be true that these fossils are of 

 Silurian age, and the line of division assumed here be the true 

 one, then in this country we should have, above our upper Pen- 

 tamerus limestone, a group of Silurian strata producing fossils 

 analogous to those described by Mr. Salter. For, notwithstanding 

 there may be a want of identity in species, the development of 

 types has always proceeded pari passu with the successive depo- 

 sitions over the whole globe, so far as our knowledge extends. 



Taking another view of the case, however, and regarding our 

 Hamilton and Chemung groups as equivalent in age to those 

 groups of England from which such similar fossils are obtained, 

 we must carry the Silurian limit to the base of the old red sand- 

 stone, which, indeed, appears to be the natural and true horizon 

 °f its upper termination. This view we think is sustained when 

 w e regard the totality of the species, and the palasontological evi- 

 dence on which we "found subdivisions. For the whole United 

 States, this would be attended with less difficulty than any other 

 *node of subdivision. Still we must re ard the fact of the intro- 

 duction of the genus Productus, and some other forms allied to 

 those regarded as Carboniferous types, at the time of the cornife- 

 rous limestone ; and the increasing number of these forms, as we 

 ascend, till the higher part of the Chemung group bears a close 

 analogy with the yellow sandstones of Ohio and Indiana, which 

 are regarded as of carboniferous age. 



These remarks are made in order to call attention to the 

 f act, that there is still room for doubt and discussion on this point, 

 and that on palfcontological evidence, there is no well-marked 

 hne of separation, separating what may on one side be termed 

 Silurian, and on the other, Devonian.] 



