49 PALAIONTOLOGY OF NEW-YORK. 



unlike to those of the Niagara group, and the fauna presents com- 

 paratively little contrast. The same genera, and very similar species of 

 Corals and Bryozoa occur in both groups. Among the Brachiopoda, the 

 Orthides of the two groups have so much resemblance that they have 

 been confounded one with the other. The Strophomena are of the same 

 character, but we have other and more numerous forms in the Lower 

 Helderberg group. The Spirifera of the Niagara and of the Lower Hel- 

 derberg formations are not readily distinguishable in several species, 

 and the Rhynchonelle and Merista equally resemble each other, and are 

 only more numerous in the later period ; while we have superadded the 

 Genus Pentamerus in P. galeatus and another similar form, and also two 

 or three genera of Brachiopoda which I have not seen in a lower 

 position. Thus in many aspects we might almost regard the Lower 

 Helderberg as a repetition of the Niagara strata. 



It is not, therefore, between these groups that we can draw the line 

 of demarcation for the Silurian and Devonian systems. Shall the advent 

 of the Oriskany sandstone, with its Spirifer of dichotomizing costas, be 

 the division ? Or shall we look for some more marked and more readily 

 defined and recognized feature for the distinction between what are 

 regarded as two great geological systems ? 



Thus far in our progress we have not recognized among our fossils 

 evidences of one great class of animals, the Vertebkata, which is re- 

 presented for the first time, so far as we yet know, in the Upper Hel- 

 derberg group, or, doubtfully, in the upper members of the Oriskany 

 sandstone*. 



We find also that a great physical change has preceded the Upper 

 Helderberg deposits ; an oscillation or sinking of a zone of the ocean 

 bed, by which the sediments of this period were allowed to spread over 

 the great western area, equally with those which preceded the Lower 

 Helderberg period. Thus we have a great but quiet physical change, 



* A single fl-agmentof a fosBil, which was referred to an Ichthyo1lt«, is known in the Oriskany sandstone. 

 The so-called fiih-ipinei, in the Niagara gcoup (Vol.ii, Pal. N.Y.), are spines of crustaceans of the Genus 

 Ceratiocarit. 



