NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 91 



irregularly pentangular. Second supraradials less than half the size of the first, pentangu- 

 lar, with the lower lateral sides shortest, each upper side supporting a small triangular piece, 

 upon which rests the first arm plate. The interradial plates are one large and two smaller 

 to each interradius. The general outline of the lower is somewhat kite-shaped; it is 

 bounded by ten contiguous plates, but the two lower lateral sides are so crowded by the 

 bounding second radials, that the two lower sides touching the first radials are extremely 

 short ; and the two short upper sides, in contact with the second interradials, lie nearly in the 

 same line. The pair of smaller interradials are nearly as long and broad, taken together, 

 as the first interradial ; the greatest width is below the middle, and their summits reach 

 higher than the bases of the arms. The intersupraradial is similar in shape to the pair of 

 upper interradials, but smaller, and the lower side is almost zero, touching the apex of the 

 third radial. Height of cup to bases of arms, 1.1 inch ; greatest diameter (at arm bases), 

 1.7 inch. 



This species greatly resembles E. crassus Hall, but a comparison of numerous specimens 

 of each shows constant differences. The first radial is of nearly the same width as the 

 second, instead of being much wider ; the first interradial is more elongated, and the two 

 lower sides together are not one fourth the length of the two corresponding sides in E. 

 crassus. The second interradials are also more elongate, and narrower. The base shows 

 a tuberculately pentagonal form not possessed by the Waldron specimens. 



Eucalypiocrinus ornatus Hall. Wis. Geol. Rep. 1861, p. 20. Differs in no way from Hall's 

 description, except that the first interradial plate is a little higher than wide ; and our casts 

 do not exhibit the characters of the exterior. 



One of our specimens possesses a sub-central proboscis, of which three eighths of an inch 

 are preserved, exhibiting such a taper that the entire length would seem to have been five 

 or six eighths of an inch. Only the cast of the proboscis exists. The dome is depressed, 

 and the base of the proboscis is elevated about three eighths of an inch above, the arm 

 bases. This is a common species, and may be at once distinguished from E. chicac/oensis by 

 its much more depressed and generally sub-hemispherical body. This is probably the spe- 

 cies mistaken by Worthen for E. decorus, which it resembles in form while it is amply dis- 

 tinct in the details of the plates. 



Cart/ocrinus ornatus Say. This species is common, and many of the specimens exhibit, 

 by the removal of the exterior, the grooves running from the pores to the margins of the 

 plates, as shown by Hall, Pal. N. Y., II. pi. xlix. fig. 1, u. The external impression of 

 the body is sometimes extremely well preserved. 



Caryocystites ct/Zindricus Hall. Geol. Rep. Wis. 1861, p. 23 ; figured in Rep. for 1862, 

 p. 69. The ovarian aperture, not seen in the typical specimens, is very distinct in a fine 

 specimen belonging to the Chicago Academy of Sciences. 



STROPHOMENA Raf. 

 Strophomena macra W. and M. 



Plate II. figure 8. 



Shell thin, with both valves nearly plane in young specimens ; general outline semicir- 

 cular, but with a straight hinge-line projecting somewhat at the lateral angles. Ventral 

 valve very slightly elevated in the umbonal region, deeply concave nearer the border in 

 old individuals, having a long, moderately wide area with a triangular fissure ; divaricator 



