24 PALEOZOIC FISHES OF NORTH AMERICA. 
largest and most characteristic fossils. These were Placoderms, scaled Ga- 
noids, and Elasmobranchs, the first preponderating in number and size, and 
included some of the most highly specialized, largest, and most formidable of 
all fishes; for example, Dinichthys, a gigantic Coccosteus, was perhaps fifteen 
feet in length, encased in armor, and provided with formidable jaws, which 
would have severed the body of a man as easily as he bites off a radish. 
The sealed Ganoids included both the rhombiferous and cycliferous varie- 
ties, and varied in size from the chub-like Palwoniscus to the cycliferous 
Crossopterygian Onychodus, eight or ten feet in length. The Elasmobranchs 
were represented by Sharks and Chimeras, but these were far less numer- 
ous than in the succeeding age. 
The bone beds of the Corniferous limestone, in which the remains of 
millions of marine fishes of middle Devonian age are strewed over the old 
sea bottom, contain numerous stud-like, often highly ornamented, dermal 
tubercles, and occasionally fragments of the pectoral spines of Macheracan- 
thus, but almost no teeth of cartilaginous fishes. Many teeth of Onychodus 
are there, often broken and sometimes worn, as though having suffered trit- 
uration; but the limestone in which they lie was deposited in comparatively 
deep and still water, and they could only have been broken and worn by 
violence or the digestive energy of the fishes which swallowed them. These 
bone beds contrast strongly with those of the Carboniferous limestone, where 
the fish remains are nearly all of Sharks, and show that somehow during the 
interval between the central epochs of these two ages the fish life of the sea 
was completely revolutionized, the powerful Placoderms having yielded 
the scepter to the Sharks, for which the Carboniferous was the golden age. 
In the Upper Devonian (Hamilton period), when the sea from which 
the Corniferous limestone was deposited had become shallowed and its sedi- 
ments were more carbonaceous and earthy, Sharks were apparently more 
numerous than before, as we find the dorsal spines of several species of 
Ctenacanthus and the teeth of Cladodus, which doubtless belonged with them, 
and yet the Placoderms are also numerous and large. Onychodus survives 
from the Corniferous period, but in a new and peculiarly modified species 
(0. Ortoni N.), in which the large median teeth of the lower jaw were planted 
in the arch of bone which bore them instead of being set astride of it. Little 
