220 PALEOZOIC FISHES OF NORTH AMERICA. 
was held between the extremities of the mandibles in the great Crossoptery- 
gian Ganoid, Onychodus sigmoides, found in the Corniferous limestone of 
Ohio, and described’ by me. There are perhaps no facts which disprove 
this hypothesis, and it is worthy of respectful consideration, but I would 
suggest that Onychodus was very widely separated zoologically from Edes- 
tus, which must have been a Plagiostome. At least, unless the skeletons 
of huge fishes like Edestus giganteus were cartilaginous, we should find their 
bones in the rocks where their spines are so numerous. 
The structure and probable functions of Hdestus have been discussed? 
by me at some length in the notes on EH. Heinrichsi, and the conclusion is 
there reached that it is not a jaw, but the defensive dorsal spine of a Plagi- 
ostome fish. The considerations which led me to this conclusion are briefly 
as follows: 
First. Although the denticles which crown the convex border of Edes- 
tus have the general form and crenulation of the teeth of Carcharodon and 
Hemipristis, their structure is in many respects quite different, viz: the teeth 
of none of the sharks are symmetrical; one face is flattened and the other is 
more or less arched; while the denticles of Hdestus are equally arched on 
both sides. 
Second. The teeth of sharks, while having enameled crowns, have 
tumid bony bases, attached by ligament to the cartilaginous jaws and sepa- 
rating readily from them; hence they are rarely found in place in the fossil 
state. The denticles of Hdestus, on the contrary, are firmly attached to the 
bony arch from which they rise. 
Third. The form of these fossils is quite unlike that of any jaw of fish, 
reptile, or mammal known; being roughly rounded at the base, the opposite 
extremity flattened, and bordered on one side by a sharp edge; on the other 
by crenulated denticles, one of which is terminal. 
Fourth. The rounded, roughened base proves that this organ could not 
have been articulated with any bones and scarcely with cartilages, else we 
should have some evidence of co-adaptation. In this respect it resembles 
most of the dorsal spines of sharks and skates, which are implanted in the 
' Paleontology of Ohio, vol. 1, p. 299, pl. 26, figs. 1-5; pl. 27, figs. 1, 2. 
* Geology of Llinois, vol. 4, p. 350. 
