436 PALEONTOLOGY OP ILLINOIS. 



vidual variation ; but it presents distinctive characteristics by which it 

 may be easily distinguished from other forms of the genus occurring in 

 our Carboniferous deposits. It is perhaps most intimately allied to the 

 form which we have described under the name A. Keolailt, with which it 

 corresponds in the number of costse and the more or less nearly obsolete 

 condition of the posterior keel; but the lateral angles are more promi- 

 nent, the posterior face more deeply channeled, and the general outline 

 less rapidly tapering. 



Position and locality : In the lower fish-bed of the Kinderhook forma- 

 tion; Burlington, Iowa. 



Asteroptychius Keokuk, St. J. and W. 



PI. XVI, Fig. 2. 



A fragment of fin-spine, showing the upper extremity of a worn speci- 

 men apparently referable to Asteroptychius, presents the following char- 

 acters: Spine strong, slightly curved in outline and gradually tapering, 

 subtriangular in transverse section ; lateral faces moderately arched and 

 rapidly converging to the more or less sharply rounded anterior edge, 

 and occupied by relatively narrow, widely spaced costse, of which there 

 are three or four in the specimen before us, the lateral surfaces beveled 

 to the inconspicuous postero lateral angles, posterior face faintly chan- 

 neled on either side of the low, or nearly obsolete, median keel. In the 

 imperfect condition of the spine the intercostal spaces preserve no trace 

 of tubercles, appearing roughened and coarsely striated longitudinally, 

 and from the same cause the character of the denticles of the postero- 

 lateral angles cannot be determined. The pulp cavity is large, occupy- 

 ing half the lateral diameter of the spine. The specimen was probably 

 at least three inches in length, and perhaps .15 inch in breadth one inch 

 from the apex. 



The specimen described above, however imperfect its state of preser- 

 vation, is unmistakably distinct from the associated forms in the same 

 deposit, while it differs from A. St. Ludovici in the rarer costation of 

 the lateral faces, and from the Chester form, A. tenuis, by its apparent 

 shorter, more robust and rapidly tapering outline. The intercostal 

 spaces are relatively broader and deeper than in A. trianguluris, N. and 

 W., of the Upper Burlington limestone, with which it is closely allied 

 in form. 



Position and locality : Bare in the Keokuk fish-bed ; Warsaw, Illi- 

 nois. 



