EASTMAN : ^ CARBONIFEROUS SHARKS. 85 



the line of insertion, and but 17 along the line of fracture where the 

 section shown in text-figure No. 11 ^ is taken. The costa3 increase in 

 number by bifurcation, and diminish gradually in width on approach- 

 ing 'the posterior margin. Pari passu with the diminution in width 

 of the costse, the transverse ridges which cross them become less and 

 less elongated, until in the ^atest formed costae 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 costae 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 

 J-ave 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. 111., Vol. II., p. 126. 



Plate XIIL, Figure 3. 

 1866. Leptacanthus C?) occidentalis Newberry and Worthen, ibid., p. 116, Plate XII., 



Figure 2. 

 1875. Acondy [acanthus occidentalis St. John and Worthen, op. cit. Vol. VI., p. 433. 

 1883. Ctenacanthus gracillimus St. John and Worthen, op. cit., Vol. VII., p. 238, 



Plate XXIV., Figure 1. 

 1889. Acondijlacanthus 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 (?) occidentalis " by Newberry and Worthen in Vol. II. 

 of the Pala3ontology of Illinois are only worn or imperfect examples 

 of the same species as Ctenacanthus gracillimus N. and W., likewise 

 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 



