FISHES OF THE CARBONIFEROUS SYSTEM. 207 
eight; the number being increased below through bifurcation. The sum- 
mit is acute, the unornamented base very short and abruptly pointed; the 
denticles extend from the summit two-thirds of the entire length. 
From Mr. William McAdams, of Alton, Ill, and Mr. H. A. Wheeler, 
instructor in Washington University, Saint Louis, I have recently obtained 
much better specimens of this spine than I had when I described the species 
in the Illinois report."| These show the summit and base, both of which 
were before unknown; the former is acute, the latter abruptly contracted, 
the unornamented portion remarkably short. In a spine of eight inches in 
length the costee reach within five-eighths of an inch of the end on the an- 
terior border and within about an inch behind. 
To the description of this species in the Illinois report I added the fol- 
lowing remarks: 
We have referred these spines to Leptacanthus with much doubt, as there seems 
to be little probability that they are generically identical with those from the Oolite, 
which Agassiz first designated by that name. It is true the general form is similar, 
but the striated or confusedly costate surface of the typical Leptacanthi must have 
given them an aspect widely different from that of these spines, so uniformly and regu- 
larly ribbed throughout. The suspicion that these differences are generic¢ is also 
strengthened by the fact that up to the present time no teeth have been found, either 
in the Lower Carboniferous or Jurassic rocks, which have been referred to the 
same genus; and it is scarcely probable that, while the fin rays of these ancient 
Placoids have been met with in considerabie numbers, the much more numerous teeth 
could have been overlooked. We infer, however, from the figures and descriptions of 
Professor McCoy (British Paleozoic Fossils, p. 633, Pl. 3%, Figs, 13, 14 and 16), that 
the spines which he calls Leptacanthus junceus and L. Jenkinsoni are generically iden- 
tical with ours; the latter species being distinguishable from that before us only by 
the less perfect regularity of costation near the base, greater spacing of the denticles, 
and the striation of the surface—fair specific differences. If, then, Professor McCoy’s 
Carboniferous fin spines are properly referred to Leptacanthus ours should be so; but 
for the reasons given above it seems in some degree probable that the reference of his 
specimens to that genus was unwarranted. The resemblance which the spines before 
us and those of_Professor McCoy bear to some of the more slender and compressed 
forms now included in Ctenacanthus, such as Ct. distans, McCoy, and Ct. gracillimus, 
N. & W., is very marked, and is suggestive of closer relationship than has been as- 
signed them. These species of Ctenacanthus have been associated with the great 
hybodoid spines Ct. major, Ag., Ct. hybodoides, Egerton, ete., simply on account of 
their sharing with them the inconstant character of the tuberculation or pectination 
of the longitudinal costs. As we have before remarked, this is almost too variable 
and superficial a character to serve as a bond of generic union between organs of which 
1Geol. Survey Illinois, vol. 2, p. 116. 
