VEETEBEATES. 253 



GENUS DBEPANACANTHUS, N. and W. 

 DEEPANACANTHUS REVEESUS, St. J. and W. 



PI. XXIV. Fig. 5. 



Drepanacanthus reversus, ST. JOHN and WOBTHEN, 1875. 111. Geol. Surv., VI, p. 45C, PI. 

 XIX., f. 5, 6. 



A nearly perfect specimen of the above named species has lately 

 been obtained at Alton. The specimen shows nearly the complete 

 outline, but it is, unfortunately, too abraded to distinctly display 

 the ornamentation, which was very clearly made out from the frag- 

 ments upon which the original description was based. The speci- 

 men here referred to. is reproduced in the illustrations. With the 

 exception of the tip and the extreme end of the base, it is perfect, 

 and is readily distinguished from the Keokuk species previously 

 described by its stronger curvature, while the tuberculation is quite 

 different, as has been already pointed out. 



Geological position and localities: St. Louis limestone ; Alton, Illi- 

 nois, and St. Louis, Missouri. 



GENUS BATACA^THUS, St. John and Worthen. 

 BATACANTHUS ? NECIS, St. J. and W. 



PL XXV. Fig. 4. 



The collection of Mr. Cox from the Keokuk limestone affords an 

 example showing a length of some 7 centimetres from the distal 

 extremity of a medium-size spine, the uncertain relations of which 

 suggested the above specific designation. In size and general form, 

 as displayed bedded in the limestone matrix, the spine is not unlike 

 Drepanacanthus gemmatus, N. and W., of the same formation. It 

 gradually tapers and is moderately curved to a point within a short 

 distance of the extremity, where it is more rapidly narrowed and 

 suddenly bent forward. But here the resemblance to Drepanacan- 

 thus ceases; instead of the wedge-shaped transverse section and de- 

 fined posterior face, the present spine is elliptic or lenticular in 

 section, anterior and posterior angles sharply rounded, the pulp 

 cavity occupying the entire posterior half of the antero-posterior di- 

 ameter of the spine, as indicated by the crushed-in thin lateral 



