210 PALEOZOIC FISHES OF NORTH AMERICA. 
a 
be made the ground of specific distinction. It is desirable, therefore, that 
the range of variation in each species should be determined as accurately 
as possible. 
Psammopus Guiyptus, St. J. & W. 
Plate XIX, Figs. 7, 8. 
Among a large number of fish teeth obtained from the Saint Louis 
limestone at Greencastle, Ind., are several teeth of Psammodus, which I have 
referred with some hesitation to the above species. They are oblong and 
quadrangular in outline, are unusually thin, and the crown surface is marked 
by a series of undulations which give them an aspect quite different from 
that of any other teeth of Psammodus I have before seen. Of known species 
of the genus those described and figured by St. John and Worthen? are like 
these in being thin and undulate, but the peculiar roughening of the surface 
which they describe is scarcely apparent; and yet much more material 
would be needed to justify the establishment of a new species upon this 
difference. 
Section F.—FisHes oF THE Coat MEASURES. 
The limestones and shales associated with the beds of coal and espe- 
cially the cannel coals, have furnished in North America, as in Europe, a 
large number of fossil fishes. Most of these are small tile-sealed Ganoids 
allied to Paleoniscus, but with them a considerable number of much larger 
fishes of the same order have been found, such as Rhizodus and Megalichthys, 
and those belonging to the interesting family of the Calacanths. We also 
find here some Dipnoans (Ctenodus), but fewer relatively than in the rocks 
of the same age in the Old World; also a considerable number of Elasmo- 
branchs, which are represented by spines, as Ctenacanthus, Edestus, Orthacan- 
thus, ete., and by teeth, as Cladodus, Diplodus, Petalodus, ete. 
Most of the Coal Measure fishes were apparently the inhabitants of 
fresh water. They include both Ganoids and Elasmobranchs which were 
among the largest and most powerful fishes known, such as Edestus, Ctena- 
canthus, Megalichthys, and Rhizodus. In the limestones of the Coal Measures, 
1 Geol. Survey Illinois, vol. 7, p. 209, pl. 14, figs. 5, 6. 
