PLATYCRINIDZ&. 687 
larger, and wider than long, followed by numerous small pieces forming 
a protuberance, which is pierced by the anus; the opening directed laterally, 
Column small. 
Horizon and Locality.— Lower Burlington limestone, Burlington, Jowa, 
Quincy, Ill., and in rocks of the same age throughout Missouri. 
Remarks, — Of this species we obtained several very young specimens 
(Plate LXXV., Fig. 11), in two of which the arms are as yet uniserial 
throughout, in others biserial only at their tips. In these specimens the 
arms have a‘distinctly waving outline, their joints are proportionally longer, 
shaped like axillary plates, and the sides supporting the pinnules are greatly 
projecting. 
We have examined the type of P. truncatus Hall, in the Illinois State 
collection, and regard the specimen as identical with P. americanus, although 
it has apparently no ornamentation. It agrees with the latter in the form of 
the dorsal cup, the channeled suture lines, the large and deep notch between 
the two posterior radials, as well as in the arm structure. Miller's P. Broad- 
headi, in all probability, is also identical with this species; it was described 
from an imperfect dorsal cup, and neither figure nor description enables us to 
make a critical comparison. 
Platycrinus tenuibrachiatus M. and W. 
Plate LXX. Figs. 7, 8. 
1869. Mrzrx and WortuEn; Proceed. Acad. Nat. Sci. Phila., p. 168; and 1878, Geol. Rep. Illinois, Vol. 
V., p. 450, Plate 3, Fig. 4a (not 42). E | 
1881. W.and Sp.; Revision, Part II., p. 75 (Proceed. Acad. Nat. Sci. Phila., p. 249). 
Of medium size. Resembling P. americanus, but the calyx proportionally 
wider, and also more discoid. Dorsal cup basin-shaped, three times as wide 
as high; the base concave, only its upper angles seen in a side view; the 
sides rapidly spreading, very slightly curving; basi-radial and interradial ) : 
suture lines channeled. Surface of plates covered with rugose markings 
or irregular rows of nodular ridges, parallel to the outer margins of the 
plates, and forming concentric lines. 
Base large, rather flat; the median portion decidedly depressed; the a) 
interbasal sutures grooved. Radials uniformly spreading; the upper end W 
nearly one third wider than the lower; the sides a little shorter than the dq 
width of the lower face; the upper angles truncated, forming a small but 
deep notch, which is filled by a hexangular interradial plate. Facets semi- 
