10 Ctenacanthus Spines from the Keokuk Limestone. 



Aet. II.- — On Ctenacanthus Spines from the Keokuk Lime- 

 stone of Lowa / by C. R. Eastman. 



Theough the courtesy of Mr. Lisban A. Cox, of Keokuk, 

 Iowa, the writer has recently been able to study certain Selachian 

 remains obtained from the so called " lower fish-bed " in the 

 vicinity of Keokuk, and now preserved in the private collec- 

 tion of Mr. Cox. Two very perfect fin-spines were considered 

 by this gentleman to be new, as they differed from anything 

 he had ever observed from this horizon or elsewhere during his 

 long experience as a collector. It will be seen from the fol- 

 lowing that he was largely justified in his conclusions. 



The larger of the two ichthyodorulites belongs undoubtedly 

 to the genus Ctenacanthus. It preserves a length of 20"5 cm , 

 and is 2*6 cm in maximum width; possibly 1*5 or 2 cm are want- 

 ing from the distal extremity, but the base is entire. It is 

 gently arcuate in form, the anterior margin being more 

 strongly and regularly convex than the posterior ; and it is 

 laterally much compressed. Its general shape and proportions 

 agree with those of a unique spine from the same horizon, 

 upon which St. John and Worth en* founded the species Acon- 

 dylacanthus f xiphias, the chief difference consisting in the 

 ornamentation. But these authors are careful to state that 

 their specimen was much abraded, and it was referred to 

 Acondylacanthus with considerable hesitancy on that account. 

 Although admitting that if found to possess nodose costse it 

 would have to be transferred elsewhere, they concluded that 

 "in the absence of any such ornamentation and the apparent 

 smooth plain costge, its affinities are clearly with the above 

 genus [Acondylacanthus]." They also point out that ' ; the 

 typical forms of Acondylacanthus are more slender and pro- 

 portionately narrower, than the above described form." 



There seems to be every reason for believing that the type 

 of A. f xiphias and the specimen now under consideration 

 represent two examples of the same species, the differences 

 between them being only such as are due to different condi- 

 tions of preservation. In this event the name must be changed 

 to Ctenacanthus xiphias, and the specific definition will require 

 emendation, so as to include characters not observed on the 

 original specimen. 



As may be seen from the accompanying figure, there is no 

 species with which these Keokuk spines are so closely related 

 as C. denticulatus M'Coy, from the Lower Carboniferous of 

 England and Ireland. There is a remarkable resemblance to 



* Pal aeon tology of Illinois, vol. VII, p. 244, pi. xxvi, fig. 1. 



