VERTEBRATES. 257 



with the latter interpretation of their relative position, or in pairs 

 at the lateral line. We are, however, not sufficiently familiar with 

 the species of the genus to he ahle to decide to what extent this 

 asymmetrical character may he relied upon or whether it is per- 

 sistent alike in all representatives of the genus. Oraeanihus has 

 already been compared with the dermal scutes of the Rays, and a 

 rather heterogeneous variety of forms have been generically identi- 

 fied. In some particulars, as the oblique course of the tubercula- 

 tion, they recall a resemblance to the Gyracanthi, but no vestige 

 exists of the strong implanted base accompanying Gyracantlms, while 

 their general proportions and outline are widely different. Neither 

 are we prepared to affirm relationship with forms of teeth occurring 

 in the same deposits, although their supposed affinities naturally 

 suggest possible relations with Psammodus. 



Geological position and localities : St. Louis limestone ; St. Louis, 

 Mo., Alton, 111., Pella, Iowa, &c. 



ORACANTHUS RECTUS, St. J. and W. 



PI. XXV, Fig. 3. 



Dr. Hamback has obtained from the Chester formation a mere 

 fragment of a spine, showing a narrow section of what appears to 

 have formed part of the dorsal or anterior edge, which certainly 

 possesses congeneric relations with the fossils from the Keokuk lime- 

 stone, to which we gave the name Oracanthus ? obliquus (111. Geol. 

 Surv., VI, p. 477, PL 12, f. 16), although they are respectively dis- 

 tinguished by well-marked specific characters. 



The Chester specimen shows a spine with a very obtusely angular, 

 nearly straight anterior ( '?) edge, the lateral surfaces diverging at a 

 moderate angle therefrom, and studded with rudely sculptured, 

 irregular-shaped conical tubercles, ranged in closely approximated 

 vertical lines and more or less regular oblique rows, on the left side 

 rising from left to right; along the anterior ridge the tubercles be- 

 come transversely elongated, showing the same strong radiating 

 plicfc and wide vertical spacing; the right side preserves a few 

 small, scattered tubercles, belonging to two or three rows, and which 

 appear to form oblique rows ascending in the opposite direction to 

 that observed in the left side. The tubercles present a more abrupt 

 slope above, and at one part ,of the fragment they appear to be- 

 come confluent along one of the lateral costse, forming an irregular, 



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