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PLATYCRINIDZA. 711 
distinctly sloping, and those toward the anal side considerably widest, giving 
to the facets of the posterior rays an excentric position. Facets occupying 
one third the width of the plates, abruptly projecting, the faces slightly con- 
vex, the outer margin crenulated, and the median portions provided with an 
obscure transverse ridge. Costals moderately large, pentangular, rounded on 
the back, the upper angle obtuse, the upper sloping faces concave, some- 
times represented by two plates. Distichals two, as large as the costals, and 
the upper plate larger than the lower. They give off an arm from the outer 
side, and two palmars from the inner, which latter support an arm at the 
one side, and two post-palmars with two arms at the other; frequently, how- 
ever, there is no further division above the palmars. The number of arms 
varies from six to nine to the ray, and often in the same specimen; the arms 
are very stout, and taper but slightly ; their plates up to the last bifurcation 
are constricted in the middle, and the ridges above and below in well pre- 
served specimens are obscurely nodose, those of the biserial upper part 
slightly convex. Structure of the ventral disk unknown. 
Column gradually increasing in width to about 70 mm. from the end, 
whence it tapers rapidly to a point. Its entire length, as presented in a 
moderately large specimen (Plate LXXIL, Fig. 1), is about two feet from the 
crown to the filiform ends of the root. It has unusually long twists, and the 
joints increase continually in length as they grow wider ; they are oval, ex- 
cept the two or three proximal ones, which are circular. The lower end of 
the stem is provided with cirri, the upper of which have a width of 3 mm., 
the lower ones of 2 mm. at their proximal ends. The latter, which grow 
very slender, are preserved in the specimen to a length of 40 mm., but were 
evidently longer. 
Horizon and Locality. — Lower Burlington limestone, Burlington, Iowa. 
Type in the Museum of Comparative Zodlogy. 
