192 PALEOZOIC FISHES OF NORTH AMERICA. 
inches or more above its base, where it was fully an inch in width, and it 
was deeply implanted in the dentary bone. The summit is rounded, evi- 
dently worn by use, but was probably once acute. It resembles closely 
the corresponding tooth of Rhizodus Hibberti, from the Carboniferous rocks 
of Scotland, but is more compressed, the margins being slightly excavated 
to render the edges thin and trenchant, so that the section is not perfectly 
lenticular, as is the case with the laniary teeth of the Scotch species. 
The jaw before us shows nothing of the segmented condition which 
Dr. Traquair claims’ is a marked character of Rhizodus and Rhizodopsis, 
but this feature would probably not be visible unless the jaw were seen 
from the inside. The general aspect of our specimen is such, however, that 
I cannot doubt it represents a species of Rhizodus and one very closely 
allied to that which is figured in most text-books on geology. It is also 
of special interest as affording the first positive evidence of the existence of 
this great sauroid fish in the waters of North America during the Carbonif- 
erous age. The scales and teeth from the Coal Measures, to which I have 
given the names of R. quadratus, R. occidentalis, R. lancifer, and R angustus, 
are only provisionally and doubtfully referred to this genus. We may be 
almost certain, indeed, that they should not be associated generically with 
the great Lower Carboniferous fish (R. Hibbert), which has been made the 
type of the genus. The specimen now under consideration is, however, so 
much like those from Scotland, that it is even a question whether it should be 
regarded as specifically distinct. If this specimen had been obtained from 
the British Islands probably no one would hesitate to identify it with R. 
Hibberti, but on comparing the great anterior laniary with a number of fine 
teeth which I have from Scotland, I find that it is more compressed than any 
of them, and is distinguished by the broad, shallow sulcus which borders 
the margin, making the slopes from the center to the edges concave. This 
is a device conspicuous in the spines of Macheracanthus, and one of which 
we make use in the construction of bayonets and some of our razors. Hence 
it has seemed to me best to consider it a new species. 
In the description of Celosteus ferox, given on another page of this 
memoir, I have alluded to its affinities with Rhizodus, and I have hesitated 
1Annals and Mag. Nat. Hist., April, 1877, p. 299, 
