346 PROCEEDINGS OF THE ACADEMY OP [1885. 



cavity of the larger plate. There are also secondary radials, but 

 those extend into free rays. 



ACROCRINUS Yandcll. 



I s ":,. Yandell, Ann r. Journ. Sci. and Arts, vol. xx (new ser.), p, 135. 



1858 Hall, Ge >1. Rep. Iowa i, PI. ii, p. 689. 



1882. Wacnsmuth, Bull, i, Illinois St. Mus. Nat. Hist., p. 41. 



Revised Generic Diagnosis. — Calyx goblet-cup or urn-shaped; 

 composed of a large number of plates, which increase in size 

 gradually from the basals up; plates thin and without ornamen- 

 tation. 



Basals two, comparatively large, either formed into a cup, or 

 thickened at the lower side and extended into a rim; sometimes 

 depressed and in form of a disk. The two plates are about equal, 

 their suture running from the anterior to the posterior side; the 

 upper side not excavated. 



Primary radials 3X5, separated from the basals by a belt of 

 numerous, small hexagonal pieces, arranged alternately in rows, 

 those of each succeeding series comparatively larger. The first 

 plate pentagonal, resting with the lower angles between the inter- 

 radial plates of adjoining fields, the upper side supporting a 

 second radial. Second radials hexagonal, more than twice as 

 large as the first, especially much wider. They abut by their 

 lower sloping sides against the upper interradials, and their 

 lateral faces rest against corresponding plates of adjoining rays, 

 except toward the posterior side, where an anal plate intervenes. 

 Their upper sides are truncate and somewhat excavated. The 

 third radials are axillary, very small, triangular, sometimes but 

 partly occupying the concavity of the preceding plate. The 

 higher orders of radials, so far as known, are extended into free 

 rays as in Platycrinus. There are 2X2X5 secondary radials, 

 which rest obliquely against the sloping sides of the triangular 

 piece. They are short but wide ; their inner sides connected by 

 a suture, the outer side partPy placed against a second primary 

 radial, filling part of its concavit}', and partly extended beyond 

 it. In Acrocr. Shumardi, the outer pair of secondary radials 

 gives oft' an arm ; the inner division bifurcates again at the second 

 plate, and supports 2X2 tertiary radials with an arm each, thus 

 giving three arms to each main division, and six to the entire ray. 

 The arm formula, however, may vary in other species. 



