96 PALEOZOIC FISHES OF NORTH AMERICA. 
The subject will be further illuminated by reference to the paper’ of 
Dr. Traquair on Dipterus, Paledaphus, Heliodus, etc., also to Van Beneden’s 
description? of Paledaphus. 
As has been mentioned, the history of the specimen described by Dr. 
Traquair is unknown, and no evidence is furnished by it of the geological 
age of the strata from which it came. From its relation to Dipterus Dr. 
Traquair infers that it came from some Palzeozoic formation. The discovery 
of a second species of the genus Ganorhynchus in the Chemung rocks of 
Pennsylvania confirms this conjecture, and renders it probable that this was 
a wide-spread form in the Devonian and Carboniferous ages. It also fur- 
nishes new evidence of the great development of this group of Dipterine 
Ganoids in the age of fishes. 
The tessellated cranium of Dipterus, so well shown by Hugh Miller and 
Pander, has little in common with that of Ceratodus, but is remarkably like 
that of Ctenodus, as will be seen by reference to the figure given on 
another page of Cfenodus Ohioensis Cope, half size linear from a specimen 
in the possession of the writer. As is remarked in the Paleontology of 
Ohio (loc. cit.), the similarity exhibited in the cranium and dentition of 
these genera is such, that new characters must be found before they can be 
satisfactorily differentiated. 
In 1858 Pander described the anterior extremity of the head of a fish 
found in the Upper Devonian rocks of Russia, which, though apparently 
distinct from Ganorhynchus, is evidently closely allied to it. In this the 
labial margin is flattened to form an arched dental plate, behind whieh on 
either side are mammillary teeth increasing in size backward. ‘This fish he 
called Holodus, and he has given® a description and figure of it. 
Later (1863) Hermann von Meyer published* a description of the 
anterior extremity of the jaw of a fish which he called Archcotylus ignotus, 
but it is evident that he had not seen the description of Holodus, for his 
specimen is generically and perhaps specifically identical with that described 
by Pander. 
. 
1 Annals and Mag. Nat. Hist., July, 1873. 
2 Bull. Acad. Belg., 2d series, vol. 17, 1864; see also Paleontology of Ohio, vol. 2, p. 62. 
8 Die Ctenodipterinen des devonischen Systems, pp. 35-60, pl. 6, figs. 1-14. 
4Palxontographica, vol. 11, p. 285, pl. 44, figs. 1-7. 
