1881.] NATURAL SCIENCES OF PHILADELPHIA. 311 



of the same form and proportions ; succeeded by three plates in 

 the second, three or four in the third, and a number of irregular 

 small plates above. Axillary pieces three or more, which, with 

 the upper interradial pieces, connect the lower portions of the arms 

 with the body. 



Vault depressed, slightl}'^ convex, composed of a great number 

 of pieces, irregularly arranged, which surround the considerably 

 larger, and generally nodose or spiniferous apical plates. The 

 central plate is always very conspicuous, and is enclosed either 

 directly by the proximal dome plates (but only in very small 

 pieces and in young specimens), or it is, more frequently, sepa- 

 rated from the others by a broad ring of small pieces, which 

 also separate the proximal plates from each other. The first 

 radial dome plates are large, isolated and pushed near the margin 

 of the ventral disk. Anal opening on top of a short proboscidiform 

 protuberance, which in Devonian species is placed more or less 

 excentric, while in all species from the Subcarboniferous it is 

 strictly lateral and located within the horizon of the arm-bases. 



Column round, very strong, composed of alternately thicker 

 and thinner joints, with large lateral cirrhi toward the lower end, 

 and an enormous root ; articulating faces of all columnar joints 

 strongly marked with radiating lines ; central canal very large 

 and decidedl}" pentalobate. 



Geological Position, etc. — Jfeg'^s/ocr^n^/s ranges from almost the 

 base of the Devonian to the Burlington limestone, where it 

 becomes extinct. It is only known from America. 



We recognize the following species : — 



1857. Megistocrinus abnormis Lyon. (Actinocr. abnormis). Geol. Rep. Ky., Hi, 



p. 479, PI. 4, figs. 1 a, b. Encrinal limest. Devonian. Louisville, Ky. 

 *1858. Megistocr. brevicornis Hall. (Actinocr. brevicomis). Geol. Rep. Iowa, i, 

 pt. ii, p. 571, PI. 10, figs. 4 a, b. Lower Burlington limest. Burlington, Iowa. 

 Sya. Actinocr. superlatus Hall, 1858. Geol. Rep. Iowa, ii, p. 572. 

 Syn. Actinocr. minor Hall, 1858. Ibid., p. 573. 

 1862. Megistocr. crassus White. Proc. Bost. Soc. Nat. Hist., ix, p. 17. Lower 



Burlington limest. Burlington, Iowa. 

 1862. Megistocr. depressus Hall. 15th Rep. N. York St. Cab. Nat. Hist., p. 134. 



Hamilton gr. Western N. York. 

 1850. Megistocr. Evansii Owen and Shum, (Type of the genus). Actinocr. Evansii. 

 Jour. Acad. Nat. Sci. Phila. (new ser.), vol. i, pt. ii, p. 68; Megistocr. 

 Evansii, 1852, U. S. Geol. Rep. Iowa, Wise and Minn., p. 591, PI. 5 A, figs. 

 3 a, b. Lower and Upper Burlington limest. Burlington, Iowa. 

 Syn. Megistocr. plenus White. Proc. Bost. Soc. Nat. Hist., ix, p. 16. 

 Syn. Megistocr. parvirostris Meek and AVorth. Proc. Acad. Nat. Sci. Phila., 

 p. 165; Geol. Rep. 111., vol. v, p. 396, PI. 6, fig. 7. 



