58 PALEOZOIC FISHES OF NORTH AMERICA. 
take the name of either of them. By other geologists it has received various 
names, and has been regarded as the equivalent of each of several strata, dis- 
tinct and somewhat widely separated in the east. The first geological corps 
of Ohio called it simply the black shale; Prof. E. B. Andrews, the Ohio 
black shale; Prof. Edward Orton, the Ohio shale; Prof. E. T. Cox, State 
geologist of Indiana, the New Albany black shale, ete., and it has been re- 
garded as the equivalent of the Marcellus, and sometimes of the Genesee 
of New York. In fact it is neither, but rather both, and it also includes the 
western extension of the Portage and Hamilton shales. This is shown by 
the fact that in different localities it has yielded fossils of all these horizons, 
viz: Clymenia complanata, Rhynchonella limitaris, Styliola fissurella, Discina 
lodensis, Lingula spatulata, ete. 
In Ohio the Huron shale is from three hundred to four hundred feet 
in thickness, contains from ten to fifteen per cent. of carbonaceous matter, 
and it is the principal source of petroleum and gas in New York, Pennsyl- 
vania, Ohio, and West Virginia. In most localities the formation is very 
barren of fossils; often in good exposures nothing being discoverable but 
obscure impressions of sea-weeds, which are thickly spread over the surface . 
of the layers, and doubtless furnished the greater part of the carbonaceous 
matter. 
In 1866 Rev. H. Hertzer, of Delaware, Ohio, found in caleareous 
septaria that abound near the base of the Huron shale some large bones 
which formed the nuclei of such concretions. These were submitted to 
me by him at the meeting of the American Association at Buffalo in 1866, 
and I recognized them as the plates and bones of large Placoderm 
fishes, up to that time undescribed. During the succeeding year Mr. 
Hertzer pursued his search for these fossils with great enthusiasm and 
success, obtaining nearly the entire bony structure of the great armor- 
clad fish which I described in the first volume of the Paleontology of 
Ohio under the name of Dinichthys Hertzeri. In this fish, while the gen- 
eral structure is similar to that of the other species of Dinichthys since 
described, the characteristic and distinguishing feature is the denticulation 
of the borders of the maxillary and mandible; in the others these borders 
are sharp edges, that play on each other like the blades of shears. 
