PLATYCRINIDiE. 729 



in such a manner, that the direction of the arms is at right angles to the 

 trunks. The second palmars of the inner sides support two orders of post- 

 palmars, the lower one giving off one arm, the upper two. One of the latter 

 arms branches again in its biserial stage so that there are four arms to each 

 cluster. Similar sets of arms are given off from the fourth order of brachials 

 at both sides of the trunks, and also from the distal end. 



The plates of the ventral disk are comparatively large and rather promi- 

 nent ; the posterior oral more elevated than any of the rest, and slightly 

 excentric, leaning to the posterior-side; the four others somewhat smaller; 

 all sharply angular and sometimes spiniferous at their summits. The cover- 

 ing pieces of the disk are quite regularly arranged in two rows. Interambu- 

 lacral plates : 3, 2, 1 ; the middle one of the first range almost as large as 

 the posterior oral, and the corresponding plate of the anal side even la°rger. 

 Column rather small for the species, the long diameter of the joints "not 

 more than once and a half that of the shorter one ; the proximal joint cir- 

 cular, and those succeeding it increasing moderately in length. 



Horizon and Locality. — Upper Burlington limestone, Burlington, Iowa, 

 and Sedalia, Mo. 



Type in the (Worthen) Illinois State collection, Springfield. 



i?marfo. — This species departs in its arm structure from all other 

 known Platycrinoids, and also is readily identified by the form of the dorsal 

 cup. There is no other species known from the Burlington hmestone in 

 which the arm facets are directed downward, and in which the calyx rests 

 upon the projecting edges of the radial facets; nor any in which the basal 

 disk is so small proportionally as in this species. It is extremely rare, and 

 was always recognized with ease by the Burlington collectors; but one 

 specimen has ever been found showing the structure of the arms, and that is 

 the crushed specimen in our own collection, which we have figured. 



92 



