292 TRANSACTIONS OF THE [MAR. 15, 



they may be distinguished by being more narrow, less curved 

 and having the summit prolonged into a relativel}^ slender and 

 acute point. In <S'. tumidus the spine is broader and more 

 curved and the dorsal prominence is more elevated and were 

 perfectly bilobed. From the smaller spines of Stethacanthus found 

 in the same beds and described in this paper (S. compressus) 

 these may be distinguished by their much greater size, lesser cur- 

 vature, greater relative thickness and different form of the dorsal 

 prominence. 



In the same beds with the spines of Stethacanthus productus 

 are found certain anomalous fin-spines, which are at first sight 

 very different from Stethacanthus, and yet while different in form 

 they are composed of the same peculiar osseous material and 

 have the same smooth, iinornamented surface, show the same 

 want of symmetry and have a similar cleft or sulcus along the 

 back. They are about four inches long by one inch or more in 

 "width, robust, straight, triangular in outline with the base slop- 

 ing backward. The cleft along the posterior margin is similar 

 to that of several species of Stethacanthus and reaches quite to 

 the summit, which is obtuse. The margins which border the 

 posterior fissure are of unequal width, making the spines un- 

 symmetrical and showing that they were not set on the median 

 line. As I have shown, all the spines of Stethacanthus hitherto 

 described were attached to the pectoral fins, the bases of which 

 were inserted in its posterior clefts. In like manner, as it seems 

 to me, the short, straight spines I have described were connected 

 with the other paired fins, the ventrals. It is scarcely probable 

 that we shall ever find in the Burlington limestone the complete 

 fins of Stethacanthus, as we have done in the Waverley shales of 

 Ohio, and so will always want the demonstration which the Ohio 

 specimens furnished, and yet the structure of these short, 

 straight spines is so entirely what it would have been if the ven- 

 tral fins of Stethacanthus had been provided with spines, that I 

 think we may fairly infer that such was their character. 



Formation and locality, Burlington limestone, Burlington, 

 Iowa. Type in the cabinet of Mr. W. F. E. Gurley, of Danville, 

 Illinois. 



Stethacanthus compressus, n. sp. 

 PI. XXIII., figs. 3-4. 



Pectoral spines of small size, three to five inches long, one 

 inch wide at base, very much compressed, extremity long, 

 pointed, dorsal prominence relatively large, rising at a high 

 angle, oval, flattened, with acute edges. Beneath this promin- 

 ence the spine is compressed to an acute edge. 



