290 Field Columbian Museum — Geology, Vol. II. 



hexagonal interradial plate, and this supporting a single radial, from 

 which the arms take their origin. Arms unknown." 



A detailed description of specimens from the Chicago Area is as 

 follows: Calyx narrowly turbinate or trumpet-shaped; arm bases 

 prominent with constrictions between ; surface of the plates smooth ; 

 sutures situated in small, but well-defined furrows; base dicyclic. 



Infrabasal plates five, equal in size, pentagonal, higher than wide. 

 Basals five; height and width about as 4:3; four are. hexagonal and 

 the posterior one is heptagonal, being truncated to support the anal 

 plate on its distal edge. The infrabasals and basal plates together form 

 a deep cup, its base apparently about the size of the upper joints of the 

 column. It expands very moderately until the middle of the basals is 

 reached, from which point the expansion is quite rapid to the arm bases. 

 Radial plates five, very much thickened in the middle of their distal 

 edges to form the prominent facets for the attachment of the arms ; 

 these facets occupy about half the width of the plate. The right 

 posterior radial is smaller than the others, on account of the radianal 

 plate being interposed between the anal plate and the right anterior 

 radial ; it rests on two of the basals and supports the right posterior 

 radial ; it is quadrangular in form. The anal plate is situated in line 

 with the radials and rests on the truncated distal side of the posterior 

 basal; it joins the left posterior radial on one side and the right pos- 

 terior radial and the radianal on the other. 



The natural casts conform in a general way with the outline of 

 the outside of the plates, with the exception of the base, which ter- 

 minates in a point, and the region of the arm bases, which is quite con- 

 stricted on account of the thickening of the distal portions of the 

 radial plates. The greatest diameter of the natural casts is at the 

 middle of the radials, but that of the outside of the plates is at the arm 

 bases. 



This species was originally described from the Niagaran shales at 

 Waldron, Indiana, and while the Chicago specimens do not agree in all 

 respects with the original description, there seems to be little doubt 

 that they should be referred to this species. In regard to the size of 

 the infrabasals Hall's description and figure do not agree. Moreover, 

 many characters, some of which, such as the size and position of the 

 anal plates, are of generic importance, are incompletely described 

 or are omitted altogether by Hall. 



Locality : The specimens from the vicinity of Chicago consist of 

 natural casts with the accompanying molds. The species was col- 

 lected by the author in the spoil heaps along the Chicago Drainage 

 Canal near Lemont, Illinois. The specimen used for the illustration 



