PLATYCRINID^. 669 



to near tlie top of the first row of interradial pieces, which curve abruptly 

 inward, so as to form with the other plates a flat surface at the top. The 

 summit plates slightly convex. Anus excentric. 



Horizon and Locality. — Lower Burlington limestone ; Burlington, Iowa, 

 and at the same horizon in Hannibal and Sedalia, Mo. 



Remarks. — This species may be readily recognized by its extremely thin 

 plates, conical base, narrow and deep horse-shoe-shaped facets, long, slender 

 arms, eight to the ray, and the waving suture lines between the arm j^lates. 



We regard P. elegans, described by Hall in the Boston "Journal of Natural 

 History," as a young Platycrinus i^lanus ; the type has uniserial arms, and 

 a round stem, which clearly shows that it is an immature specimen. The 

 figure which Hall gives of it in Bull. 4 of the N. Y. State Cabinet is crushed 

 and somewhat misleading with regard to the form of the dorsal cup, which is 

 not turbinate as described, but the sides are almost parallel along the radials, 

 and spread upwards but little, if any. We figure on Plate LXIX, Fig. 

 2d, a similar specimen, but somewhat more mature, which already has inter- 

 locking plates in the upper half of the arms, while in the lower part the arms 

 are zigzag, and their joints long and uniserial. The joints of the young stem 

 are remarkably long and circular, resembling those of a young Comatula in 

 the Pentacrinus stage, being at 3 cm. from the calyx almost as long as wide, 

 and there are no internodal joints interposed between them. Another speci- 

 men in our collection, which in all other respects agrees with the preceding 

 one, has uniserial arms throughout like Hall's P. ekgans. 



Platycrinus Agassizi W. and Sp. (nov. spec). 

 Plate LXIX. Fig. 4. 



Syn. P. planus (?) W. and Sp. (not Owen and Sudm.); Geol. Rep. Uliuois, Vol. VIII., p. 188, Plate 16, 

 Kg- 8. 



Smaller than P. plamcs. Dorsal cup elongate, the sides along the radials 

 almost cylindrical, very slightly expanding at the middle, its lower margin 

 produced into a sharply edged projecting rim, which in form resembles an 

 upper stem joint, but is larger and tripartite. Plates very thin, without 

 ornamentation, and the suture lines without grooves. 



Basals forming a shallow basin, in height from one fourth to one third the 

 length of the dorsal cup. Radials quadrangular, a little longer than wide; the 

 plates somewhat thickened longitudinally, giving to the facets some promi- 



