INVEETEBEATES. 295 



This species differs in its arm formula from all other Poterio- 

 known at this time, and its depaiture from the normal 

 structure may prove of generic importance. 

 . Position and locality: St. Louis limestone, Monroe county, 111. 



No. 2,456, Illinois State collection. 



POTERIOCRINUS siMiLis, Worthen. 



PI. XXX, Fig. 12. 



Po/eriocrinus simiHs, WORTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 23. 



Body small, basin-shaped, twice as wide as high to the top of 

 the radial series. The plates of the body are all slightly protu- 

 berant in the center, and depressed at the angles. 



Base slightly depressed and basals concealed by the first columnar 

 joints. 



Subraclials about as wide as high, hexagonal and heptagonal. 



Eadials wider than high, with a well-defined suture between them 

 and the brachials. Brachials on the two posterior rays nearly 

 three times as long as wide, constricted in the middle and angular 

 above, supporting on their sloping sides the two divisions of the 

 rays. In the right antero-lateral ray the brachial is only about 

 twice as wide as high, its upper angle projecting so as to form a 

 short node. 



Arms two to each ray on the three rays visible, composed of 

 rounded joints that, at first, are nearly twice as long as wide, but 

 they gradually become shorter above. They all project laterally, 

 and support strong pinnules, that are given off alternately from the 

 longest side of the arm joints. The lateral projections of the joints 

 give a zigzag arrangement to the arms. 



Only one anal plate is visible in the specimen, and this is placed, 

 as is usual in this genus, between two of the subradials, and under 

 the left side of the right posterior radial. 



Column round, and at its upper extremity composed of joints of 

 nearly equal thickness. 



This little crinoid is related to Pot. Columbiensis, from the same 

 locality, but differs from that in its more depressed base, and in 

 the slightly protuberant character of its body plates. 



Position and locality : Chester limestone, Monroe county, 111. 



No. 2,446, Illinois State collection. 



