56 PALEOZOIC FISHES OF NORTH AMERICA. 
closely approximated. It may also be said that while on some large slabs 
of limestone we have found apparently most of the bony portions of Ony- 
chodus, among these were no plates such as belong to the carapaces of the 
Placoderms; and we have, therefore, no evidence that it has any affinity 
with Asterolepis, Coccosteus, ete. The scales of Onychodus are not unlike 
those which were attributed by Hugh Miller to Asterolepis; but we now 
know that these scales really belong to Glyptolepis, and that, so far as known, 
the body of Asterolepis was without scales. 
Since the above notes! on Onychodus were written, in 1873, two other 
species of the genus have been discovered, viz: O. Hophinsi, N., and O. Ortoni, 
N., both of which are noticed on the following pages. But of the first only 
the intermandibular teeth have been found, and of the second a single inter- 
mandibular arch carrying teeth; so that they add little to what was learned 
from the abundant remains of O. sigmoides in the Corniferous limestone. 
The descriptions and plates are here republished for the purpose of 
bringing together all the material yet known which can serve to illustrate 
the remarkable dentition of Onychodus and to enable the reader better to 
appreciate the comparisons which have been made between that and Hdestus. 
OnycHopus sIGMomDEs, Newb. 
Plate XXXVI, Figs. 1-4°; Plate XX XVII, Figs. 1-10. 
Onychodus sigmoides, N.; Paleontology of Ohio, vol. 1, p. 299. 
Fishes of large size; head at least eighteen inches long, composed of 
numerous angular and rounded plates, supported on a cartilaginous brain- 
box, and so imperfectly united that in the fossil state they are usually dis- 
connected and scattered. Of the head plates, the opercula are from three 
to five inches in diameter, nearly circular, but with a produced anterior 
angle. The maxillaries are triangular in outline, the anterior and posterior 
angles much produced, the lower margin nearly straight, and set with a 
large number of conical, pointed teeth. The dentary bone of the mandibles 
is often more than a foot in length, curved gently upward at its anterior 
extremity, which is rounded. Its posterior extremity is thin and flattened 
' Paleontology of Ohio, vol. 1, pp. 296-299. 
