CRINOIDS AND BLASTOIDS. 189 



the calyx is nearly cylindrical, expanding a little at the arm 

 The first radials are a third longer than wide, and their 

 upper margins generally of equal height, forming a straight 

 transverse line all around, without any truncation of the cor- 

 ners for the iuterradial plates. The articular facet is small, 

 rather shallow, facing upward, without any prominent rim. 



Arms usually six to the ray. with some exceptions: long, 

 comparatively strong and with closely packed pinnules. The 

 plates are smooth and thin, sutures not indented. The basal 

 disk has a sharp prominent circular lip for the attachment of 

 the column, forming a part of the basal cup. although having 

 the appearance of an auchylosed upper stem joint. The basal 

 cup appeal's as a single plate, no traces of sutures being visible. 

 Vault uukown. Specimens all light colored. 



The specimen figured is of about medium size, some being 

 considerably larger, others smaller. It has only 5 arms in three 

 of the rays, and the upper margins of the radials are not so 

 uniformly rectilinear as in most of the others, but otherwise it 

 fairly represents the form herein described. 



Whether this is really P. planus is uncertain. The difficulty 

 arises from the confusion in which the literature of that species 

 is involved. Owen and Shumard described it in the report on 

 the Geology of Wisconsin, Iowa and Minnesota, p. 587, from 

 the Lower beds at Burlington as a thin plated, elongate spe- 

 cies, with subturbinate base, long radials with small and deep 

 articular facets, and the point of attachment for the column 

 slightly excavated. They figure on PI. 5a. Fig. 4, two speci- 

 mens : one of moderate size answering the description fairly well 

 except that there is no excavation for the column ; the other of 

 a very much larger form, which has been described by Prof. 

 Wortheu as P. pmtteni. They also speak of specimens having 

 been seen nearly two inches in diameter, which would be an ex- 

 traordinary size for any Lower Burlington Platycriuus. Hall, 

 in the Report on the Geology of Iowa. 1858, Vol. II, Pt. '2, p. 

 533. PI. 8, Fig. 6, figured and described the species with some 

 doubt as to the specific identity of his specimen. His figure 

 shows very plainly that he was describing a specimen of P. 

 halli Shumard. from the Upper Burlington, a very different 



