62 PALEOZOIC FISHES OF NORTH AMERICA. 
Prof. O. C. Marsh has a number of large fragments of Placoderm fishes 
obtained from the Huron shale at the Falls of the Ohio; they apparently 
represent one or more new species, but are too imperfect for satisfactory 
description. 
From the. Moscow shale, Kashong Creek, Yates County, N. Y., Mr. Ber- 
lin H. Wright has obtained a large and fine spine of Ctenacanthus, which is 
distinguished from all other described species of the genus by its perfectly 
straight form, as well as by minor details. It is figured and described’ with 
the name of Ct. Wrighti, and is noticed on another page of this memoir. 
In the Hamilton rocks of Lowa, which are chiefly limestone, a consider- 
able number of fish remains have been found, a few of which have come 
under my observation. Of these the most important are two species of 
Rhynchodus* apparently distinct from those found in Ohio, viz, . occiden- 
talis N. and R. Greenei, n. sp., which will be found characterized in the 
present monograph; the latter obtained through Mr. Thomas A. Greene, of 
Milwaukee. From Mr. Greene I also have a fish spine of a peculiar and 
interesting structure, which I have made the type of a new genus, Hetera- 
canthus, of which a description will be found on another page. These speci- 
mens were found in the quarries of hydraulic limestone near Milwaukee, 
and with them were fragments which indicate the presence of a varied fish 
fauna in that formation. 
From Mr. A. F. Tiffany, of Davenport, Iowa, I have received a fine 
specimen of Ptyctodus calceolus,? N. & W. The specimen from which the 
species was originally described was from the Hamilton beds of Calhoun 
County, Ill. From the same horizon in Rock Island-County, IL, I have a. 
large number of what seem to be the teeth of a distinct species of Ptyctodus, 
but they may be only a dwarf form of the same. These latter teeth are 
small—an inch in length by a quarter of an inch broad—and show all the 
variation of form represented by Pander’s figures.* 
The zoological relations of Ptyctodus still remain uncertain, but there is 
* Described in the Annals of the New York Academy of Sciences, vol. 1, p. 192. 
‘First described in Geol. Survey of Illinois, vol. 2, 1866, p. 106, pl. 10, fig. 10, as Rinodus calce- 
olus ; and later in the Palxontology of Ohio, vol. 2, p. 59, pl. 59, figs. 13, 134, 13>. 
4Die Ctenodipterinen der Deyonischen System, pl. 8. 
