358 
The American Geologist. 
May, 1896 
Even here, however, our investigation does not altogether 
fail, for within a few years the clue has been found again on 
this side of the Atlantic and in Silurian strata of probably 
somewhat greater antiquity than any of those already men¬ 
tioned. But in entering on the details of this last chapter in 
our story it will be well to dwell for a moment on the physical 
conditions of the time. I have pointed out the fact that in 
Lower Devonian periods communication was interrupted be¬ 
tween the European and the Appalachian seas. But in the 
earlier (Upper) Silurian era this communication was at least 
comparatively free. Evidence of this is found in the great 
number of species that are common to the two areas. The 
passage-way of the communication may have been direct, that 
is over the present Atlantic, or it may have been circuitous 
across western America and Asia. In either case the demands 
of paleontology will be satisfied. 
In 1883 the writer, while examining some beds of sandstone 
and shale in Perry county, Pennsylvania, discovered many 
specimens of fish remains which on further study were found 
to be closely related to the pteraspids of the English Ludlow 
beds and evidently represented the same family, which had 
not been previously recognized on this continent. They were 
described by him under the name of Palaiaspis* and scarcely 
differ except in minor details from their confamiliars of 
Europe. Their presence in the Appalachian area can be ex¬ 
plained by the recognition of a water communication as above 
shown. But the stratum in which they occur is probably 
lower than that in which the oldest of the English pteraspids 
is found. The relationship may be showm as follows: 
English 
Ludlow, upper 
‘ ‘ lower 
Wenlock 
The Ludlow beds are correlated 
lower Helderberg of North America, 
exist in England between these and the Wenlock beneath 
them, which are on indisputable evidence correlated with the 
Niagara. In Appalachia however the thick colored marls of 
Appalachian 
Lower Helderberg 
Onondaga, Salina 
Niagara 
by their fossils with the 
But no gap is known to 
*Quart. Jour. Geol. Soc., 1883. 
