1897.] NEW YORK ACADExMY OF SCIENCES. 287 



species, which he called P. trigonalis and described and figured 

 it in the Report of the Geological Survey of Illinois (Yol. YII., 

 p. 259, PL XXIV., fig. 4). The considerations which influenced 

 him in separating these spines from Oracanthus are not very 

 plain, since they have so much the form, structure and orna- 

 mentation of that genus. However, nothing positive can be 

 said for or against the separation until more and better speci- 

 mens shall afford means of comparison. The distinction which 

 he suggests on page 419, Volume VI,, viz., that the spines of 

 Fnigeacanthus are more like dermal scutes than dorsal spines is 

 also true of some specimens of Oracanthus Milleri^ as shown 

 from the plates and text of Mr, J. W. Davis' description of that 

 species. 



In volume VI. of the Report of the Geological Survey of 

 Illinois (pp. 477, 478) Mr. St. John describes two additional 

 species of Oracanthus, viz., 0. ? obliquus and 0. consimilis. The 

 specimens on which the descriptions are based are only frag- 

 ments, and later (Vol. VII., p. 255) he unites 0. consimiUs with 

 O. vetustus of Leid3\ In this last mentioned volume (p. 257) 

 he describes and figures (PI. XXV., fig, 3) still another species 

 of Oracanthus^ which he calls 0. rectus. The specimen is but a 

 fragment and hardly aflfords material for comiDarison, but it 

 comes from the Chester limestone and may ver^- well be distinct. 



In the same volume, (PL XXIV.) is figured a spine of Ora- 

 canthus that before the discovery of that which forms the sub- 

 ject of this memoir would have been regarded as a splendid one, 

 inasmuch as it shows the termination and the ornamentation 

 much more distinctly than anj- before known. This he regards 

 as identical with Leidy's 0. vetustus which is perhaps true, as 

 that is probably a ver}' variable species, and yet the question 

 ma\^ be open to doubt, the ornamentation is so different in char- 

 acter and arrangement. 



A spine of Oracanthus has recently been sent to me for ex- 

 amination by Mr. William F. E. Gurley, of Danville, 111., which 

 throws a flood of light on the structure of the spines of this 

 genus and shows that we have had a very imperfect idea of their 

 real nature. This is so well shown in the present figure, PL I., 

 fig. 3, that no lengthy description is needed. It is practically 

 complete, only a small portion of the tip being wanting, and it 

 shows, what has been before unknown, the entire base of the 

 spine which was buried in the integument. This is nearly as 

 long as the exposed portion and is an elongated arch or half 

 tube of bone which must have served as a firm support to resist 

 all strains upon the spine from the front backwarcl. The orna- 

 mented portion is below thickly crowded with relativel}' large 



