1881.] NATURAL SCIENCES OF PHILADELPHIA. 305 



than the second, hexagonal or heptagonal, with an obtuse upper 

 angle supporting the secondary radials. The latter usuall}'^ 

 consist of 2 X 10 plates, the upper row, in some species, curving 

 outward and supporting the free arms, in others straight, axillary, 

 and supporting the higher orders of radials, which give off the 

 arms from every second plate, and alternatel}'^ from opposite 

 sides. The arms in either case are simple or branching, long, 

 slender, rounded, decreasing in width upward. They are from the 

 body up composed of two series of small alternating pieces, and 

 the bifurcating takes place after the arms have passed into a 

 double series of interlocking pieces. 



Pinnules slender, closely abutting. 



Interradial plates numerous ; the first one large, generally 

 hexagonal, supporting two plates in the second, and two In the 

 third range, with several series aboA'e, each containing from two 

 to three plates, which gradually decrease in size toward the 

 summit. The posterior side much wider ; first anal plate hep- 

 tagonal, equal in form to the first radials and in line with them, 

 followed b}'- three plates in the second series, and by a large 

 number of small plates above. 



Interaxillaries numerous. Vault depressed, from moderatelj^ 

 convex to almost flat; composed of small, irregularly arranged, 

 smooth pieces, among which the apical plates are indistinctly 

 represented. Anus subcentral, either in form of an opening 

 through the vault, or in some species probably extended into a 

 small tube. 



Column large, round, central canal moderately wide and round. 



Geological Position., etc. — Periechocrinus occurs from the Upper 

 Silurian to the base of the Subcarboniferous (Burlington Limest.), 

 and is found both in America and Europe. 



We recognize the following species : — 



*186I. Periechocrinus amplus Meek and Worth. (Actinocr. Pradocr. amplusl. 



Proc. Acad. Nat. Sci. Pbila., p. 133; also 1868, Geol. Rep. 111., iii, p. 470, 



PI. 16, fig. 2. Upper Burlington limest. Burlington, Iowa. 

 1843. Periechocr. articulosus Austin. Ann. and Mag. Nat. Hist., xi, p. 204. Upper 



Silur. Dudley, Eng. 

 «1830. Periechocr. Baylii de Vemeuil. fPradocr. Baylii) Bull. Soc. Geol. de France 



(ser. ii), vol. vii, p. 184, PI. 4, figs. 11 a, b, c. Devonian. District Sabero, 



Spain. 

 *I878. Periechocr. brevimanus Angel. (Actinocr. brevimanus) Iconogr. Crin. 



8uec., p. 6, PI. 18, fig. 12. Upper Silur. Gothland, Sweden. 



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