158 ECHINODERMATA PELMATOZOA SUB-KINGDOM in 



Streptocrinus, W. and Sp. (Ophiocrinus, Ang. non Salter nee Semper). 

 Plates of the dorsal cup as in Cyathocrinus. Ventral sac composed of about 

 eight rows of hexagonal plates, some of which are deeply folded transversely, 

 so as to produce well-marked depressions on the surface. Arms slightly 

 bending downward; their ambulacral grooves narrow, and provided with 

 covering pieces only. Silurian ; Gottland. 



Tenarocrinus, Bather. Dorsal cup broad, and composed of thin plates ; the 

 truncated posterior B supporting a large IRA. The right posterior E com- 

 pound; its superradial shifted to the right; but the inferradial, notwith- 

 standing its slanting position, gives no support to the ventral sac. The latter 

 is very large, longitudinally folded, and covered with transverse rows of 

 pits. Column round, with very wide axial canal. Silurian ; Dudley, 

 England. 



Parisocrinu-s, W. and Sp. Dorsal cup obconical, composed of five IB, five 

 B, and five R, with an anal and radianal ; the latter two together supporting 

 the long, cylindrical, ventral sac. Upper edges of the R excavated so as to 

 form a narrow semicircular facet for the reception of the first costals ; 

 number of the latter very variable among the rays. Arms multibrachiate, 

 divergent, and filiform at their tips. Column round. Devonian ; Germany. 

 Sub-Carboniferous ; North America. 



Atdestocrinus, W. and Sp. Dorsal cup elongate, always more or less con- 

 stricted along the suture line between the basals and infrabasals. IB five, 

 large, forming an almost solid ovo-cylindrical body. B five, long and narrow ; 

 R five ; four of them equal and arm-bearing, the anterior one less than half 

 the size of the others and not arm-bearing. Costals 4x4, the uppermost 

 axillary, and supporting two main arms ; armlets given off on alternate sides 

 from every second plate. Anal interradius as in the preceding. Burlington 

 and Keokuk Groups ; Mississippi Valley. 



Sphaerocrinus, Roemer, and Achradocrinus, Schultze. Of these only the 

 structure of the calyx is known. The dorsal cup in both forms is globular, 

 composed of very thin plates, and the arms are pierced by a dorsal canal. In 

 the first-named genus IRA and HA are both represented, and it has an 

 eccentric anal opening which is directed upwards. But in the second RA is 

 wanting, and the position of the anus is strictly lateral, it being situated in 

 the suture line between the posterior radials. Middle Devonian ; Eifel. 



Codiacrinus, Schultze. Calyx small, resembling a poppy head. IB five ; 

 B five ; R five ; the plates within each circlet equal to one another, and 

 laterally in contact. Devonian ; Germany. 



Lecythiocrinus, White. Arrangement of calyx plates as in the preceding, 

 but with wider radial facets ; these are directed upwards instead of obliquely 

 outwards, and are provided with transverse ridges and well-defined ligamentous 

 fossae. Coal Measures ; North America. 



Family 9. Poteriocrinidae. Roemer. 



Base dicydic. Infrabasals five, sometimes hidden by the column. Basals five ; 

 radials jive, with transverse articular ridges. Anal and radianal generally repre- 

 sented ; and lower plates of the tube frequently entering into the dorsal cup. Ventral 

 sac large ; elongate or balloon-shaped in the earlier forms, small and conical in the 

 later ones ; it is composed of hexagonal plates, having the angles perforated by pores. 



