224 J. Hail on the Primordial Fauna, 
pee Silurian rocks as well as some of them in Devonian. 
‘he same is true of the Cephalopoda enumerated. 
Tetradium is known in the second fauna of the Lower Silu- 
rian rocks, and in the upper part of the Hudson River group at 
the west. Dictyonema is a genus known from Lower Silurian to 
Devonian strata. 
Graptolithus proper extends to the Clinton group of New 
‘ York; and the same is true of Reteograptus. Thamnograptus 
known in the Quebec rocks only; while the typical form 
Beg on i ag occurs in the Potsdam sandstone, and, likewise, 
in three other species, in the Quebec rocks, 
_ We find, therefore, in the other genera except trilobites, very 
little satisfactory evidence, on which to rely in the present state 
of our knowledge, for determining the position of these strata. 
In the present discussion, it appears to me necessary to go fur- 
ther, and to inquire in what manner we have’ obtained our ad 
ent ideas of a reo, or of any successive faune. I old 
that in the study of the fossils themselves there were no means 
of such determination prior to the knowledge of the stratigrapb- 
ical relations of the rocks in which the remains are inclosed. 
There can be no scientific or systematic paleontology without @ 
stratigraphical basis. Wisely then, and independently of theo 
ries, or of observations and conclusions elsewhere, geologists ? 
this country had gone on with their investigations of struct al 
geology. The grand system of the Professors W. B. and H. p 3 
ogers had been wrought out not only for Pennsylvania and 
