Upper Devonian Placoderms of Ohio. — GJaypole. 857 
older forms, from which we must infer the absence of some 
volume of the records intermediate between that found in 
Russia and this earlier one from Great Britain. These are 
yet to be sought and probably found elsewhere. 
But in spite of this still existing gap the paleontologist is 
warranted in his deduction that the two faunas, the Middle 
Devonian of Russia and the Lower Devonian of Scotland and 
England, are genetically connected and that only further 
search is needed to bring the records to light and complete 
the genealogical tree of these early fishes, waiving of course 
the possibility that the volume has been wholly destroyed. 
No difficulty is encountered in taking the next step. In this 
we cross the frontier between the Devonian and Silurian sys- 
tems. In the uppermost beds of the latter, which in England 
lie conformably under the lowermost Devonian strata, we find 
fish remains in great abundance of the same type as those last 
mentioned, but of different species. Cephalaspis is no longer 
seen, but Pteraspis and one or two kindred forms represent 
the fishes and are so like those of the lowest Devonian tile- 
stones immediately over them as to leave no doubt of their 
relationship. The upper and lower Ludlow shales contain 
this early record of ichthyic life; the former abundantly, but 
the latter has as yet yielded a single specimen of Cyathaspis 
(Scaphaspis) ludensis to show that at that remote date fishes 
had come upon the stage of being in the English seas. But 
of any earlier forms no British evidence is forthcoming. The 
Llandovery shales and sandstones with Pentajnerus oblongus, 
the taxonomes of the Clinton of New York, have yielded no 
trace of fishes. 
Returning now for a moment to the Russian area we find 
some indications that Cephalaspis was there of rather ear- 
lier date, for in strata in the Isle of Oesel, which yield the 
common English molluscan fossils of the Ludlow rocks, Eieh- 
wald reports a Cephalaspis ( Thyestes verrucosus) whose con- 
geners do not appear in England until we rise to strata char- 
acterized by their Mollusca as Devonian. Hut in spite of this 
the English Lower Ludlow with its solitary Cyathaspis (Sca- 
phaspis) is seemingly the very lowest horizon to which in the 
Old World fish t'ussils have been traced. 
