FISHES OF THE CARBONIFEROUS SYSTEM. 195 
Order ELASMOBRANCHII. 
Genus PHYSONEMUS. 
The genus Physonemus was named (though never described) by Prof. 
L. Agassiz in 1837,’ but the spines to which the name was applied were 
well known, so it was recognized and a generic description given by McCoy.’ 
He also described two species, one, that which had suggested the generic 
name to Agassiz (Ph. subleres), and a new one, which he called Ph. arcuatus. 
Another spine, named and described by Agassiz* as Onchus hamatus, having 
the strong curvature of Physonemus, has been referred to that genus by Mr. 
J. W. Davis.* He has also added another species to the list, (Ph. attenuatus, ) 
which, though having the strongly recurved form of the type species of the 
genus, is too imperfectly preserved to admit of any positive assertion 
as to its generic relations. From the peculiar form of the base, indeed, it 
seems more probable that this is but one branch of the forked spine of 
Cladacanthus, Ag 
In all that has been written by Europeans on the spines of Physonemus it 
has been supposed that they were set on the dorsal line, with a very strong 
backward curvature; but in a description of Physonemus gigas® I have called 
attention to the characters which indicate that this spine, as well as the one 
described by McCoy, was curved forward instead of being straight or curved 
backward, like most of the dorsal spines of Elasmobranch fishes ; supposing 
the line of large, stellate tubercles set along the concave border, as well as 
the direction of the oblique line joining the plain and ornamented surfaces, 
to indicate that the curved border was anterior. These characters are 
shared by the spines upon which I have based the description of Drepana- 
canthus,® illustrated in the figures of D. gemmatus.’ It is therefore not certain 
that these last-mentioned spines and Physonemus arcuatus, McCoy,* should not 
be united under the same generic name; but a comparison of specimens 
will alone suffice for deciding this question. 
! Poissons Fossiles, ete., vol. 3, p. 176. 
5 Geol. Survey Illinois, vol. 4, p. 373. 
2 Brit. Pal. Fossils, p. 638. ®Tbid., vol. 2, p. 120, 
3Poissons Fossiles, etc., vol. 3, p. 9, pl. 1, figs. 7, 8. 7Ibid., p. 123. 
4Mon. Fossil Fishes Carboniferous Lime- 8Tbid., vol. 4, p. 373, pl. 2, fig. 17- 
stone series Great Brit., p. 370, 
