PLATYCRINIDZ. 673 
across the arm; in the upper [lower] part the arm plates are proportionally 
longer, and extend entirely across, but separating very slightly the two 
adjacent plates on the opposite side, making a single range of plates, with 
their longer faces alternately on opposite sides.” This is exactly the con- 
dition of the arms in every young Platycrinus when it enters the biserial 
stage. 
Platycrinus Sampsoni 8. A. Miter. 
Plate LX X. Fig. 10. 
189]. S. A. MitteR; Geol. Surv. Missouri, Bull. 4, p. 18, Plate 1, Fig. 11. 
A large species of a very unusual form. Dorsal cup cylindrical almost 
throughout its full length, perhaps a little narrower at the upper end; the 
base broadly truncated. Plates thin and smooth, the suture lines very 
slightly grooved. The basal cup resembles a very low disk with nearly 
erect sides; it is very broadly truncated at the bottom, the sides slightly 
expanding ; the upper faces form an almost straight line around the cup, 
the angles being so obscure as to be scarcely perceptible ; height of the cup 
equal to a little more than one third the length of the radials; the inter- 
basal suture lines barely visible. Radials of unequal size, some of them 
much wider than others, the widest ones about once and a half as wide as 
Jong, the narrow ones almost twice as long as wide; the sides parallel. 
Facets surrounded by a projecting rim; they are narrow, moderately deep, 
and directed upwards, All other parts unknown. 
Horizon and Locality. — Upper Burlington limestone, Burlington, Iowa, 
and Sedalia, Mo. 
Type in the collection of Mr. Sampson. 
fiemarks. — Described by Miller from a natural cast, but the species is so 
remarkable that it is readily identified. We possess of it a nearly perfect 
dorsal cup from which our description was made. 
