340 rROCEEDINQS OP THE ACADEMY OF [1881. 



the middle line, growing thin toward the edges. In some species 

 the arms become full}- an inch wide, while in others the increase 

 in width is comparatively insignificant. In some species, every 

 arm joint from both sides is extended, in others only every 

 second, third or fourth joint, while the intermediate ones remain, 

 narrow. In these arms the wider joints appear in the form of 

 lateral spines, which begin short, but gradually increase to a 

 considerable length. Ambulacral furrows deep ; pinnules similar 

 to those of Batocrinus. 



Interradial plates one to three, the first large, extended to the 

 middle portions of the third primary radials, the upper ones very 

 small. Anals, one in the first, three in the second series, with a 

 few additional plates above. Interaxillarles none. 



Dome elevated, inflated, generally larger than the calyx, com- 

 posed of nodose or spiniferous plates ; the apical plates more 

 prominent and larger ; the large centre piece occupying the very 

 centre of the vault ; posterior side with a shallow depression filled 

 by small anal pieces. 



Anal tube strong, more or less excentric, extending beyond the 

 infolding arms, and frequently bent abruptly to one side ; the 

 middle portions more or less inflated, top and base of almost 

 equal width, the upper end provided with a small opening. 



Column round, composed of very wide pieces alternating with 

 much smaller ones, the former frequently with sharp edges ; 

 central canal small. 



Geological Position, etc. — Eretmocrinus occurs at the age of the 

 Burlington and Keokuk groups, in America only. 



We recognize the following species : — 



186 1. Eretmocrinus attenuatus Hall. (Actinocr. matuta var. attenuata) Desc. 

 New. S|). Crin. p, 14. Upper Burlington li > est Burlington, Iowa. 



1860. Eretmocr. calyculoides Hall. (Actinocr. calyouloides) Supp. Geol. lowrt, p. 



17, Photogr. PI. 3, figs. 2-5; Meek and Worth., 187H, Batocr. (Eretmocr.) 

 calyculoides Geol. Rep. 111., v, p. 368. Upper Burl, limest. Burlington, 

 Towa. 

 1S61. Eretmocr. carioa Hall. (Actinocr. carica) Desc. New. Sp. Crin. p. 10; Meek 

 iind Worth, 1873, Batocr. (Eretmocr.) carica, Geol. Rep. 111., v, p. 368. 

 Lower Burlington limest. Burlington, Iowa. 



1861. Eretmocr. clio Hall. (Actinocr. clio) Desc. New. Sp. Crin. p. 1; also 



Bost. Journ. Nat. Hist., p. 262, Photogr. PI. 3, fig. 7; Meek and Worth., 1873, 

 Batocr. (Eretmocr.) clio, Geol. Rep. II'., v, p. 3fi8. Lower Burlington 

 limest. Burlii'gton, Iowa. 



