PLATYCEINID.E. 707 



and radials covered with comparatively few, irregularly distributed, rather 

 large and prominent nodes, which sometimes almost take the character of 

 spines. The costals, distichals, and lower palmars have from two to three 

 nodes, which are somewhat smaller than those of the calyx, and transversely 

 arranged, while the biserial arm plates have but one, placed near the inner 

 end, forming together with adjoining ones two rows along the back of 

 the arm. 



Basals forming a pentagon, with a circular, rather deep concavity, wide 

 enough to admit the two upper stem joints. Interbasal sutures frequently 

 invisible ; the basi-radial and interradial suture lines slightly grooved. Eadi- 

 als hexangular, the upper ends of the plates one third wider than the lower. 

 Facets very large, facing outward, shallow, transversely elliptical ; the upper 

 end slightly excavated for the ambulacral passage. Costals large, triangular, 

 occupying the full width of the facet. There are normally five arms to the 

 ray (exceptionally four or six), one division with three arms, the other with 

 two, the former having two distichals to the inner side ; arms of moderate 

 size, their lower ends curving slightly downward ; the arm joints rather long. 

 Ventral disk from a third to a half higher than the dorsal cup ; the outer 

 ends of the ambulacra slightly protruding ; the plates highly elevated, their 

 summits crowned by two or three sharp tubercles. Orals large ; the posterior 

 one pushed to the anterior far enough to be in a straight line with the two 

 postero-lateral ones. Ambulacral plates large, arranged quite regularly in 

 two rows. Interambulacral plates from four to five, except at the anal side 

 where the middle one of the first row is much larger than the corresponding 

 plate of the other sides, and followed by a great number of small tumid 

 pieces, which form a large, conspicuous, subcentral protuberance containing 

 the anus. The two plates at the sides are twice as long as wide, and longer 

 than the middle one. Column distinctly twisted ; the joints slightly angular 

 around the margin ; their long diameter equal to twice the shorter one. 



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



Remarks. — The dorsal cup of this species is less discoid than it appears 

 in specimens with the arms preserved. The arm facets enter deeply upon 

 the plates, and the lower parts of the arms are directed horizontally, often 

 with a downward tendency. 



