VEETEBEATES. 241 



tubercles of the posterior ribs are similarly beautifully ornamented 

 and distinguishable by their more oval transverse outline, relatively 

 smaller size and acuminate crest which rises abruptly over the 

 superior margins. The plain surfaces are striato-punctate longitudi- 

 nally. The type specimen attains a length of 11.5 centimetres, the 

 proportions as also the form being well represented in the illustra- 

 tions. 



The species above noticed is one of the handsomest of the genus. 

 The specimen figured is the most perfect, but other fragments have 

 been obtained indicating larger-sized spines equally strongly marked 

 by the distinctive features of the species, and which readily dis- 

 tinguish it from any allied form known to us. In the deflected 

 costae along the posterior margins it bears some resemblance to 

 Ctenacanthus deflexus of the St. Louis limestone, but is otherwise 

 markedly distinct from that species, as will be apparent on compar- 

 ing the illustrations given of each respectively. This is one of the 

 many interesting discoveries of the remains of fishes in our Coal 

 Measure strata made by Mr. Alexander Butters, in whose honor the 

 specific name is given. 



Geological position and locality: Lower Coal Measures (roof shales 

 over coal No. 5, Illinois general section) : Carlinville, Illinois. 



GENUS ACO^DYLACA^THUS, St. J. and W. 



ACONDYLACANTHUS BECTUS, St. J. and W. 

 PL. XXVI. Fig. 2. 



Dorsal ray of small size, slightly arched along the sharply-rounded 

 anterior edge, moderately compressed laterally, and very gradually 

 tapering throughout its length. Posterior face deeply grooved and 

 traversed by a faint median keel, striato-punctate, and bordered by 

 the relatively thick walls of the postero-lateral angles, which latter 

 are rounded and bear close along their inner edge a row of compara- 

 tively strong, closely approximate, laterally compressed, downward- 

 hooked denticles, which extend apparently from the apex (where 

 they are worn obsolete) to the dorsal line, below which the pulp 

 cavity issues. Lateral surfaces moderately convex transversely, 

 rounded into the postero-lateral angle, and occupied by eight or more 

 close, rounded costse separated by narrow intercostal furrow, those 



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