15-2 ECHIXODERMATA PELMATOZOA SUB-KINGDOM in 



radial. On the left it supports small plates of the ventral sac, without the 

 intervention of a larger plate. Ordovician ; St. Petersburg. 



Hybocystes, Wetherby. Arrangement of calycine plates the 

 same as in Hybocrinus, but differing in the arm structure. 

 Three of the rays bearing primitive arms composed of but 

 few joints ; ambulacral furrows passing from the ventral to 

 the dorsal side of the arms, and continued upon the surface 

 of the E, The two other rays are without arms, and the 

 ambulacra follow the surface of the calyx, passing down so 

 far as to enter the basals - Ordovician ; Kentucky. 



Petersburg. calyx Ba&i'ocninus. Volborth. Like Hoplocrinus, but the right 



seen from the anal . * 



side (after ore- posterior and the anterior ray without arms ; apparently 

 inferradials only are developed. Ordovician ; St. Petersburg. 



Family 2. Anomalocrinidae. Waclismuth and Springer. 



Base monocydic. Radials very irregular in form, the right posterior and left 

 antero-lateral ones compound, all of tliem laterally in contact. Ventral sac small, 

 tubular ; its lowermost plate resting in the angle formed by the superradiol to the right, 

 and the upper end of the simple radial to the left. Arms uniserial, bifurcating' at 

 intervals. Pinnules or small armlets given off from each arm-joint on one side only, 

 as far as from one bifurcation to the next, ivhen they change to the opposite side. 

 Ordovician. 



Anomalocrinus, M. and W. B five. Unique in that the large simple R of 

 the posterior ray is often longitudinally bisected in the median line. A small 

 quadrangular supplementary piece enclosed within the basal ring. Tegmen 

 composed of large plates. Column strong. Ordovician ; North America. 



Family 3. Heterocrinidae. Zittel (emend. W. and Sp.) 



Base monocydic ; basals five ; one or more of the radials compound. The infer- 

 radial of the right posterior ray supporting to the right the superradial, and to the left 

 the ventral sac. Anals absent. Arms uniserial. Ordovician and Silurian. 



Heterocrinus, Hall (Stenocrinus, W. and Sp.) Crown sub-cylindrical, calyx 

 small, arms long. Three of the R compound ; the others simple, and shorter. 

 The inferradial of the posterior ray has the form of an axillary, supporting to 

 the left the ventral sac, and to the right the superradial. Arms comparatively 

 stout, giving off long branchlets at intervals, which often branch again. 

 Column very large, pentagonal, and quinquepartite ; the lines of suture inter- 

 radial in position. Ordovician ; North America. 



Ectenocrinus, S. A. Miller (Heterocrinus, W. and Sp. non Hall). Like the 

 preceding, but the arms formed of continuous series of syzygies comprising two 

 plates each, the epizygals giving off long armlets. Column of moderate size, 

 round, and tripartite ; axial canal large and pentalobate. Ordovician ; North 

 America. 



Ohiocrinus, W. arid Sp. Differs from Heterocrinus in the details of its arm- 

 structure, and in the form of the ventral sac. Arms ten, long, and giving off 

 from every third or fourth joint long filiform armlets, which extend to the full 

 height of the main arms. Ventral tube resting upon the left sloping side of 

 the right posterior inferradial (RA\ and ascending spirally with the adjacent 



