358 The American Geologist. May, i»96 
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. Hut in entering on the details of this last chapter in 
our story it will he well to dwell for ;i 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. Hut 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 OA T er 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 18N3 the writer, while examining some beds of sandstone 
and shale in Ferry 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 Palceaspis* 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 lie shown as follows: 
English Appalachi an 
Ludlow, upper Lower Helderberg 
" lower 
********* Onondaga. Salina 
Wenlock Niagara 
The Ludlow beds are correlated by their fossils with the 
lower Helderberg of North America. But no gap is known to 
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 
♦Quart. Jour. Geol. Soc., 1883. 
