FISHES OF THE CARBONIFEROUS SYSTEM. 221 
integument of the back and have a roughened base and bony structure, 
with various forms of enameled denticles on the margin. 
Fifth. If the spines of Edestus were attached to the head as modified 
jaws and the homologues of the rostrum of Pristis, the base would present 
some evidence of anchylosis with the bones or cartilages of the head; 
whereas it is rounded, as though it had been buried in soft tissue. Again, 
the rostrum of Pristis is only partially ossified, while the spines of Hdestus 
are composed of dense bone ; and, further, the denticles of the rostrum in 
Pristis ave set in alveolar cavities, from which they escape and are scattered 
about in the decay of the animal. We often find these denticles in the Cre- 
taceous marls, but almost always isolated, like the sharks’ teeth which occur 
with them. On the contrary, the denticles of Hdestus are inseparably united 
with their bony bases, and they are perfectly preserved together. 
Finally, if each spine of Hdestus was one of a pair attached to the 
snout, like the rostrum of Pristis, Xiphias, or Celorhynchus, they must have 
been entirely separated, for they bear no marks of contact, and they would 
certainly have been unsymmetric 1. We are therefore driven by the bilat- 
eral symmetry of Edestus to conclude that it was not one of a pair, but that 
it stood alone somewhere on the median line, either as a homologue of the 
‘ntermandibular arch of Onychodus, the dorsal spines of Chim ra and Hybo- 
dus, or of the caudal spines of Trygon. 
The suggestion of Miss Hitchcock that Edestus is an intermandibular 
bony arch carrying teeth is not incompatible with its bilateral symmetry ; 
but we here meet the difficulty already suggested, that Onychodus, the only 
fish known which had such an intermandibular arch of bone, was a scaled 
Ganoid allied to Polypterus and has left abundant bones beside its inter- 
mandibular arch. In Onychodus sigmoides of the Corniferous limestone, and 
0. Hopkinsi of the Chemung group, the teeth are not anchylosed to the 
arch, are almost always found detached, and the sides of the arch are com- 
pressed between the extremities of the mandibles. In 0. Ortoni, of the 
Huron shale, the teeth are implanted in the bony arch as a post is set in 
the ground, and the arch is not distinctly impressed by the extremities of 
the mandibles. The type specimen of 0. Ortoni is yet unique, and we know 
nothing of the other parts of the fish which bore it. It is, of course, not 
