222 PALEOZOIC FISHES OF NORTH AMERICA, 
impossible that this singular form of dentition might have been borrowed 
by some Plagiostome which used it to accomplish a similar function; but 
no facts are yet known to warrant this supposition. 
Edestus Davisi is more like the intermandibular crest of Onychodus than 
are the other species of the genus. It is much more curved and the arch 
of bone from which the denticles rise is laterally compressed or longitudi- 
nally grooved. ‘Taken by itself, it renders the suggestion of Miss Hitchcock 
quite plausible. But it cannot be taken by itself; for wherever that species 
goes, E. minor, E. Heinrichsi, and E. giganteus must follow, and while we 
‘can imagine a fish ten feet long with an arch of bone like E. Davisi held 
between the extremities of the mandibles, it requires a much greater stretch 
of the imagination to conceive of a shark of such size that this relatively 
insignificant organ was twenty inches long and seven or eight inches wide. 
Certainly such a monster would seem very much out of place in the lagoons 
of the coal marshes. Again, E. Heinrichsi is nearly straight, a foot long, 
rounded and massive at one end, thin and acute at the other; but the suc- 
cession of denticles, was by additions to the acute end, which must have 
been behind, for if it was situated in the symphysis, the blunt, rounded end 
would have formed the apex of the arch of the lower jaw; a condition of 
things scarcely comprehensible. 
If, now, we transfer this spine to the position of the post-dorsal fin, and 
bury it in the soft parts, all except the denticles, the elongation backward 
by the successive addition of sheaths and denticles becomes intelligible and 
natural. 
There are some other features in this fossil which require notice, viz: 
There is no distinct line of demarkation between an exposed and a buried 
portion, such as we find in most of the defensive spines of sharks, unless, as 
seems probable, all the shaft was buried and only the denticles exposed. 
Another peculiarity is the absence of the medullary cavity found in most 
dorsal spines of Sharks. This is quite conspicuous in the spines of Hybodus, 
Ctenacanthus, etc.; but in the Rays the spines are solid, and there is little 
distinction between the exposed and buried parts. ‘The exceptional char- 
acters just mentioned need not, therefore, be considered incompatible with 
the view that these fossils are dorsal spines. 
