PLATYCRINID&. 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 
r 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. Radi- 
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. ‘he 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. 
