42 
PALAEONTOLOGY OP 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 Strophomence are of the same 
character, but we have other and more numerous forms in the Lower 
Helderberg group. The Spiriferce of the Niagara and of the Lower Hel- 
derberg formations are not readily distinguishable in several species, 
and the Rhynchonella 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 costse, 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 Vertebrata, 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 fragment of a fossil, which was referred to an Ichtliyolite, is known in the Oriskany sandstone. 
The so-called fish-spines, in the Niagara geoup ( Vol. ii, Pal. N.Y.), are spines of crustaceans of the Genus 
Ceratiocaris. 
