; EASTMAN: CARBONIFEROUS SHARKS. 85 
the line of insertion, and but 17 along the line of fracture where the 
section shown in text-figure No. 114 is taken. The costz increase in 
number by bifurcation, and diminish gradually in width on approach- 
ing the posterior margin. Part passu with the diminution in width 
of the coste, the transverse ridges which cross them become less and 
less elongated, until in the latest formed costz they are almost tuber- 
cular. None of the transverse crests are striated, and they are spaced 
at approximately regular intervals apart. Occasionally the decussa- 
tions belonging to several consecutive cost extend across the inter- 
costal spaces and become fused. 
The anterior margin is rounded and bears a somewhat wider longi- 
tudinal ridge than the rest, from which only one or two bifurcations 
are given off. The posterior walls of the pulp cavity appear to be 
slightly swollen, and the cavity itself is slightly open throughout the 
entire length of the part preserved. No other examples of this species 
have as yet fallen under the writer’s observation. 
Formation and Locality. — Kinderhook Group ; Iowa or Illinois. 
Ctenacanthus gracillimus N. and W. 
1866. Ctenacanthus gracillimus Newberry and Worthen, Pal. Ill., Vol. II., p. 126. 
Plate XIII., Figure 3. 
1866. Leptacanthus (?) occidentalis Newberry and Worthen, ibid., p. 116, Plate XIL, 
Figure 2. 
1875. Acondylacanthus occidentalis St. John and Worthen, op. cit. Vol. VI., p. 433. 
1888. Ctenacanthus gracillimus St. John and Worthen, op. cit., Vol. VIL. p. 288, 
Plate XXIV., Figure 1. 
1889. Acondylacanthus occidentalis J. S. Newberry, Monogr. U. S. Geol. Surv., 
Vol. XVI., p. 206, Plate XXV., Figure 6. 
Although this is one of the most abundant ichthyodorulites occurring 
in the St. Louis limestone, and a number of more or less perfect ex- 
amples have been obtained, confusion exists regarding both its generic 
and specific titles. There can be no doubt as to the correctness of 
Messrs. St. John and Worthen’s conclusion that the spines described as 
“ Leptacanthus (2?) occidentalis” by Newberry and Worthen in Vol. II. 
of the Palzontology of Illinois are only worn or imperfect examples 
of the same species as Ctenacanthus gracillimus N. and W., likewis< 
published in Volume II. (1866). These authors had then to decide 
which of Newberry and Worthen’s figured specimens should be selected 
as the type, and by which of the two names the species should be 
VOL. XXXIX. —NO.3. 8 
4 
