PLAXrCRINID^. 665 



trigonal, the outer end of the first distichal barely touches the radials. Dis- 

 tichals placed obliquely, about as long as half their width, the axillary 

 obtusely angular above. There are from six to nine higher orders of 

 brachials above the costals, of which the proximal ones consist of two 

 jjieces each, the succeeding ones of three each, all giving off from their 

 axillaries at one side an arm, and from the opposite side the next order 

 of brachials. The plates of the different orders decrease in width upward, 

 but not in length, those of the higher orders being almost as long as wide, 

 while those of the lower orders in large specimens are almost twice as wide 

 as long,- In young specimens all the brachials are proportionally longer, and 

 the bifurcations extend to nearly the full length of the rays ; while in the 

 adult they rise to only half way np. Arms biserial from the second plate 

 after the axillary, short for the size of the calyx, and all rise to the same 

 height. They are composed of moderately long joints, and each one has 

 a well defined lateral projection for the support of a jDinnule. 



Form of the ventral disk unknown ; we observed, however, from a 

 crushed specimen that it is composed of convex pieces apparently of a simi- 

 lar arrangement to that of P. Halli, except that the ambulacral pieces are 

 more projecting and comparatively smaller. The large interambulacral plate 

 of the first row at the anal side is somewhat bulging outward, and wider than 

 long. Column elliptic and twisted, the long diameter of the joints being at 

 25 cm. from the calyx twice as long as the short one. The joints increase 

 gradually downward to three times the length of those at the top, and near 

 the base of the stem give off from their sharp ends two rows of small cirri, 

 which follow the twist. 



Horizon and Localiitj. — Upper Burlington limestone ; Burlington, Iowa. 



Ttjpe in the Museum of Comparative Zoology. 



Remarks. — In the branching of the arms this species approaches the 

 genus Eucladocrinus, but while in the latter the successive orders of brachials 

 form tubular appendages of the calyx closed from all sides, in P. incomptus 

 they retain the characteristics of arm plates, being provided with open 

 furrows. The type specimen shows the bifurcations only to the third axil- 

 laries above the costals, and it was supposed the species had but four arms 

 to each side of the ray ; while in fact it has from seven to ten, or as many 

 as twenty arms to the full ray in some specimens. 



