PLATYCRINID^. kjj 



distinctly sloping, and those toward the anal side considerably widest, givin<. 

 to the facets of the posterior rays an excentric position. Facets occupying 

 one th.rd the width of the plates, abruptly projecting, the faces slightly cont 

 vex, the outer margin crenulated, and the median portions provided with an 

 obscure transverse ridge. Costals moderately large, pentangular, rounded on 

 he 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 

 crownto 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 gro^i: 

 very slender, are preserved in the specimen to a length of 40 mm., but were 

 evidently longer. 



Horizon and Zocalit^;. -Loy^ev Burlington limestone, Burlington, Iowa 

 %^e m the Museum of Comparative Zoology. 



