270 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



apparently independent dorsal plates, that overlie one another in 

 a scalelike manner, but this determination needs to be substantiated 

 on better preserved material. Along the median line of the arm 

 ossicles there lies ventrally a single column of ventral plates, that 

 bear spines and externally cover the radial canal of the water vas- 

 cular system. The lateral branches from the radial canal to the 

 ambulacral podia do not penetrate the substance of the arm ossicles 

 as in living ophiurids, but lie between each pair of vertebrae. The 

 oral skeleton consists of five interradially placed mouth-corner 

 pieces. Each one of these is made up of three elements, the two 

 outer of which are united into pairs" (pp. 60-61). 

 Contains the following species : 



0. flexilis Meek and Worthen. Keokuk. 



0. asper Miller. Keokuk. 



0. barrisi (Hall). Burlington. 



0. confragosus Miller. Keokuk. 



0. demissus Miller. Keokuk. 



ONYCHASTER FLEXTLIS Meek and Worthen. 

 Text figs. 37 to 42. 



Onychastei* flexilis MEEK and WORTHEN, Geol. Surv. Illinois, vol. 3, 1868, p. 526, 

 figs. A-D; Proc. Acad. Nat. Sci. Philadelphia, vol. 21, 1869, p. 83; Geol. Surv. 

 Illinois, vol. 5, 1873, p. 510, pi. 16, figs. 3a-3Z. ZITTEL, Handb. Pal., vol. 1, 

 1879, p. 444, fig. 314. MILLER, N. Amer. Geol. Pal., 1889, p. 264, fig. 374. 

 SCHONDORF, Jahrb. nassauisch. Ver. Naturk., Wiesbaden, vol. 62, 1909, 

 pp. 47-61; vol. 63, 1910, p. 240. I. B. J. SOLLAS, Philos. Trans. Roy. Soc. 

 London, ser. B, vol. 204, 1913, pp. 51-62, text figs. 1-4, pi. 8, figs. 1-6. 

 SCHONDORF, Jahrb. nassauisch. Ver. Naturk., Wiesbaden, vol. 66, 1913, pp. 

 97-114, text figs. 1, 2, pi. 3, figs. 3-9. SPENCER, Mon. Brit. Pal. Asterozoa, 

 pt. 1 (Palseontogr. Soc. for 1913), 1914, pp. 26, 50. 



Original description. "The interesting fossil on which we propose 

 to found this genus and species seems to differ so widely, in some of 

 its characters, from the true starfishes, as well as from the Ophi- 

 urians, as to leave doubts whether it can be properly placed in either 

 of these groups as now understood. * * * In habit and general 

 appearance it most nearly resembles the Ophiurians, from which, 

 however, it differs widely in structure. It is composed of a rather 

 small subdiscoid body, and five long, slender, rounded, flexible arms 

 or rays. In nearly all of the specimens yet found the arms are folded 

 together like the claws of a bird when grasping some small object. 

 A few of them, however, have the arms opened out more or less, so 

 as to show that they were very flexible, or capable of being moved 

 about in all directions. They usually increase slightly in thickness 

 for a short distance from the body, then taper very gradually to their 

 extremities, being about 2.50 inches in length, and 0.22 inch in 

 breadth, at the widest part. 



