38 PALEOZOIC FISHES OF NORTH AMERICA. 
Comparatively few cartilaginous fishes of the present epoch are pro- 
vided with even dorsal spines; and none, so far as known, carry spines 
on the pectoral fins. But in the Mesozoic and the Paleozoic ages the Elas- 
mobranch fishes were much more generally provided with spines, and 
it is not too much to suppose that this tendency to the development of 
organs of defense should be exhibited in spines appended to the anterio 
paired fins. We have, in fact, positive evidence that some of the sharks 
of the Carboniferous age did carry pectoral spines. 
Several of the species of Gyra- 
canthus, for example, are now gener- 
ally conceded to have been pectoral 
spines. This is shown by their want 
of symmetry, which proves that they 
were not set on the median line, and 
by the fact that they are worn off 
in various degrees, as though used 
as organs of locomotion on the sea 
bottom or shore. A series of spines 
of Gyracanthus formosus, in the eabi- 
net of Columbia College, show, 
(1) a slender, complete, evidently 
young spine ten inches long and 
Fic. 1. Spines of Machaeracanthus peracutus, 
unworn; (2) a large spine nine 
inches long, of which the summit is obliquely and smoothly worn off where 
five-eighths of an inch in diameter; (3) the base of an old and very large 
spine worn away nearly to the end of the ornamented portion. 
I have also shown in another part of this memoir that the spines called 
Physonemus Altonensis by St. John and Worthen were certainly appendages 
to the pectoral fins. This was first suspected from their obvious want of 
symmetry, and was finally proved by finding complete specimens buried in 
soft shale, in which the cleft of the convex margin is occupied by the base 
of a broad pectoral fin. 
