800 THE CEINOIDEA CAMEEATA OF NORTH AMERICA. 



b.isals. The costals very minute, sometimes comi^letely covered by the dis- 

 tichals, which are a little larger than the palmars. The latter, to the height 

 of the third plate, consist of short, transverse, single pieces, the jjlates of the 

 two lower rows being included in the calyx ; while those of the third row, 

 which are surmounted on the back by a small spine or elongate node, are 

 free. Anns biserial from the fourth plate, and composed of two rows of very 

 short pieces, which are united laterally by a zigzag suture. The arms are flat, 

 tapering, and so short that their ends do not meet upon the summit. Ven- 

 tral disk pyramidal, a little shorter than wide. Interarabulacral spaces trian- 

 gular, and to their full height covered with small longitudinal grooves in 

 which the arms rest. At the four regular sides there are four plates, three 

 of them interambulacral, arranged : 1 and 2, followed by an oral plate. The 

 two upper interambulacral pieces and the orals are strongly bevelled at one 

 side to form the sockets for the appendages. The upper parts of the four 

 smaller orals bend abruptly inward, and form a sort of platform, which is 

 almost invariably occupied by a specimen of Platyceras chcsterense, so as to 

 cover the anal opening and the posterior oral. Anal interradius a little the 

 widest, and composed of a much larger number of plates. It has also only 

 one plate between the arm bases, but this is followed by three or four pieces, 

 and numerous others which gradually decrease in size upwards. The pos- 

 terior oral is convex and semilunate, one of its sides being excavated by the 

 anal opening, which is almost central. The sockets for the appendages are 

 widest at the top, whence they taper gradually downward, and form a sharp 

 angle which enter.5 the upper end of the interdistichal below. The appen- 

 dages are long and somewhat variable in form ; as a rule, they are wider on 

 top than at the bottom, a little deeper than wide, deepest at the proximal 

 ends, and they terminate either in a sharp point or a transverse edge. 

 Column small and round. 



Horizon and LocaUtij. — Kaskaskia group ; Sloans Valley, Pulaski Co., Ky. 



Ti/pes in the collection of Professor Wetherby. 



Hemarks. — We regard Pterotocrinus sjoatulatus Wetherby as identical 

 with the above species. The two were said to differ in the form of their 

 appendages, the one being awl-shaped at the distal end, the other more 

 spatulate. Comparing the figures of the type specimens, the difference is 

 very slight, and a comparison of a large number of spines collected from the 

 dumps of the Sloans Valley tunnel shows conclusively that there are within 

 the limits of this species even greater variations than those I'eferred to. 



