224 PALEOZOIC FISHES OF NORTH AMERICA. 
symmetry of Edestus proves that if employed in this manner it must have 
been located on the upper margin of the tail or back. 
The segmented structure of Hdestus has led Dr. Woodward to compare 
it with the spines of Pelecopterus, and especially with the pectoral spines of 
this genus described by Prof. E. D. Cope,’ but the symmetry of Edestus 
forbids the acceptance of this conclusion. The pectoral spines of all fishes 
are unsymmetrical. This is plainly seen in Macheracanthus and Gyracan- 
thus, and, as I have lately shown, in the pectoral spines of Stethacanthus, 
(Physonemus) Altonensis. Pelecopterus probably had dorsal as well as pec- 
toral spines, and a comparison with them would be better grounded; but 
as that was a bony fish, the dorsal spines would have an articulation at the 
base, and would have grown at the base and not at the summit. 
In the spines of Trygon, however, we find a much closer resemblance 
to Edestus—one that seems to me to go far towards solving the problem of 
the relations and functions of these peculiar organs, and almost decides that 
they are dorsal spines. In Trygon a considerable number, sometimes five 
or six, defensive spines are set in the place of the posterior dorsal fin. They 
come into use in succession, like the fangs of venomous serpents. As the 
anterior one loses its denticles or becomes worn or broken it falls and is 
succeeded by another from behind. Yet several may be in existence and 
effective at the same time, all rising from a common segmented bony base 
which grows by additions to its posterior extremity. 
All this is true of the spines of Hdestus if we are right in locating them 
in the position of the second dorsal fin on the back or tail of a Plagiostome 
fish. 
Hence, until further light shall be thrown upon the interesting question 
of the homologies and functions of Edestus, we may regard them as the post- 
dorsal spines of large cartilaginous fishes of which the other parts are yet 
unknown, and may suppose that they were used for attack and defense like 
the spines of Trygon or Acanthurus. 
‘Geol. Survey of the Territories, vol. 2, p. 244A. 
2On the tail of Heliobatis radians, Marsh, a fresh-water ray from the Eocene Green River beds 
of Wyoming, I have seen three spines, which must have been in service at the same time; and Gunther 
(Study of Fishes, p. 190) figures a group of five spines in function on the tail of Aétobatis narinari. 
