428 PALEONTOLOGY OF ILLINOIS. 



ing in the narrow, slightly beveled space along the posterior borders ; 

 intercostal spaces usually equaling or slightly exceeding the diameter 

 of the costa?, faintly striato-punctate. The first pair of costse on either 

 side the anterior ridge bear delicate, obliquely transverse tubercles, 

 spaced by about twice their diameter, but sometimes connected by the 

 thread-like lines descending from the long inferior slope of the tuber- 

 cles ; the succeeding ridges are smooth or faintly striated longitudinally, 

 with the exception of the first and second which are sparcely occupied 

 by miuute, irregularly spaced tubercles. Posterior face slightly chan- 

 neled, median keel inconspicuous, postero- lateral margins forming prom- 

 inent rounded angles above, less obtuse below, and bearing relatively 

 strong, slightly hooked, sculptured denticles, separated by about twice 

 their diameter, perhaps more widely spaced above. Pulp cavity nearly 

 half the diameter of the spine iu the middle, long elliptical in section. 

 Base unknown. 



The spines here referred to bear striking resemblance to the Upper 

 Burlington form which we have described under the name G. Burling- 

 tonensis, so strong is the likeness, indeed, that the majority of the imper- 

 fect specimens of the two forms are scarcely distinguishable one from 

 the other. The collection of Dr. WnLLiAiis contains several more or 

 less perfect specimens of the present form, and the careful examination 

 of this material seems to justify the disposition we have here made, in 

 accordance with the apparent peculiarities enumerated in the observa- 

 tions under G. Burlingtonensis. It is not improbable more complete 

 material would show even more intimate relations between these spines, 

 or differences such as the better to serve their characterization. 



Position and locality : Not uncommon, but generally in fragmentary 

 condition, in the Keokuk fish-beds ; vicinity of Warsaw, Illinois ; Keo- 

 kuk and Bentonsport, Iowa ; Boonville and Le Grande, Missouri. 



Ctenacanthvs excatatus, St. J. and W. 



PI. XV, Fig. 4-5. 



Fin-spine of moderate length, very strong, rapidly tapering, mode- 

 rately curved, transverse section ovate, truncated posteriorly. Lateral 

 faces gently arched behind and slightly converging to the posterior 

 borders, rapidly rounded in front to the anterior margin, which is occu- 

 pied by a strong broadly rounded ridge, apparently smooth along the 

 crest with indications of transverse decussations or tubercles in the 

 lateral margins. The lateral surfaces are covered by a few very promi- 

 nent, decussated costse, spaced about their own diameter by deep 

 intercostal grooves. Posterior face broad, more or less concave or 

 deeply channeled, keel apparently inconspicuous, postero-lateral angles 



