PLATYCRINID^. 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, Iowa, 

 Quincy, 111., 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. tmncatus 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. Meek and Wokthen ; Proceed. Acad. Nat. Sei. Phila., p. 168 ; aud 1873, Gaol. Rep. Illinois, Vol. 



v., p. 450, Plate 3, Fig. 4« (not 4i). 

 ISSl. W. aad Sp. ; Revision, Part II., p. 75 (Proceed. Acad. Nat. Sci. Phila., p. 249). 



Of medium size. Eesembling 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 

 interbasal sutures grooved. Radials uniformly spreading; the upper end 

 nearly one third wider than the lower ; the sides a little shorter than the 

 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- 



