708 THE CKINOIDEA CAMEEATA OF NORTH AMERICA. 



Platycrinus Yandelli, var. perasper Shum. 



Plate LXVUI. Figs. 4a, h. 



1865. P. perasper — Shdmaed ; Catal. Pateoz. Foss. of North Anier., p. 389. 



18S1. P. perasj>er — W. and Sp.; llevisioD, Part II., p. 73 (Proceed. Acad. Nat. Sci. Phila., p. 217). 

 Sjn. P. mdobrackiatus Hall, 1861 (not 1858); Descr. New Spec, of Crin., p. 17. 



In the form of the dorsal cup, the proportions of its plates, and the size 

 of the arms, closely resembling P. Yandelli, but the nodes covering the sur- 

 face much more numerous; they are placed close together without being 

 confluent, are very prominent, and cover the dorsal surface of the arms. 

 Those upon the brachials to the last axillary, although smaller, are equally 

 well defined, and are arranged transversely in two rows of from four to five 

 nodes to each row, those upon the biserial arm plates in only one row. The 

 latter are more or less confluent, and form an undulated ridge at the upper 

 edge of the plates, which gives to the transverse sutures a waving outline. 

 This variety generally has six arms to the ray, which are slightly flattened 

 on the back and obscurely grooved along the median line. The stem twists 

 rapidly, and is composed at its proximal end of short elliptic joints. 



Horizon and Localiti/. — Lower Burlington limestone, Burlington, Iowa. 



Ty2oe in the Museum of Comparative Zoology. 



Platycrinus spinifer W. and Sp. (nov. spec). 

 Plate LXVI. Fig. 7. 



Closely resembling P. Yandelli, but somewhat larger. Dorsal cup saucer- 

 shaped, the basals proportionally large, the radials distinctly curving, their 

 facets projecting outward. Plates thin, covered with large, rather sharp 

 nodes, placed in concentric lines parallel to their margins ; there being gen- 

 erally three such lines upon the basals, and two or three upon the radials, 

 the latter arranged so as to form a diagonal line from the facet to each 

 lower angle of the plates. Similar nodes, but somewhat smaller, cover the 

 arms, everj'- plate above the costals having three nodes — the costals four to 

 five — transversely arranged, and occupying the median line of the plates; 

 sometimes, however, in the upper part of the arms, the nodes become 

 confluent and form undulated ridtres. 



Basals large, slightly convex except the middle part, which is abruptly 



