214 PALEOZOIC FISHES OF NORTH AMERICA. 
The largest fishes of the Linton group are as yet known only by frag- 
ments, and we have therefore very much to learn about them The tessel- 
lated cranium of Ctenodus has, however, been found nearly entire. This is 
a foot or more in length, showing that the fish to which it belonged was of 
considerable size; but the few teeth of the genus yet found in this locality 
are disproportionately small. 
The teeth of Diplodus are exceedingly common and some of them are 
of large size. In the largest (D. latus) the lateral denticles are broader, 
thinner, and more lancet-shaped than in any of those found in Europe.’ 
The two smaller species, which I have called D. compressus and D. 
gracilis, are hardly to be distinguished from those named by Agassiz D. gib- 
bosus; but the spines of Orthacanthus found at Linton—which belonged to 
the same fishes—form two or three species, which are apparently all distinct 
from those associated with the teeth of Diplodus in foreign localities. In 
several instances I have found the cartilaginous jaws more or less perfectly 
preserved and still bearing the teeth; these formed many rows from front 
to rear, with many teeth in a row, lying appressed like the rear teeth of 
Carcharias, ete. 
Taking all things into consideration, the Linton locality is the most in- 
structive of all our known repositories of fossil fishes, chiefly because we 
have here the history of a colony which can be read with a good degree of 
completeness; a kind of window, through which we can look into the Car- 
boniferous age, and over a limited area see everything that was taking 
place; and yet this was but a part, and a very small part, of a great whole. 
There is no doubt that the species found here once lived in a thousand other 
localities, and with them were many others of which we as yet have no 
traces. 
The nodules of iron ore contained in the coal shales on the banks of 
Mazon Creek near Morris, Ill, generally contain organic nuclei, and thou- 
sands of beautiful specimens have been obtained there. They are usually 
fragments of fern fronds, but are sometimes shells, crustaceans, myriapods, 
‘All the specimens of this species found before my description was published (Paleontology of 
Ohio, vol. 2, p. 44, pl. 58, fig. 1) were without a central denticle, and I gave that as one of its charac- 
ters; but Mr. M. C. Read has recently sent to me a large and finely preserved tooth, in which the middle 
cone, though very small, is distinctively visible. 
