HEX ACRINID A. 790 
by an obscure groove, and similar grooves, but deeper, mark the interbasal | 
suture lines. The processes upon the radials occupy the upper two thirds of | 
the plates, leaving their lower ends and the upper portions of the basals 
almost free from ornamentation. The facet which supports the costals is 
surrounded by a thickened collar, from which three — exceptionally two — 
ridges or series of nodes pass downward; the two outer ones in the direction 
of the lower angles of the plates, the other following the median line. 
Basal cup large, occupying over two fifths the height of the calyx. | Wy 
Radials considerably wider above than below, their length about equal to 
their width at the upper end; the lower faces very little convex, the upper 
slightly excavated; the facet directed upwards, occupying one third the 
width of the plates; the limbs somewhat inflected but not notched. Costals 
two, rounded on the back, the lower very short and linear; the second 
a little longer and axillary, its sloping upper faces at right angles. Dis- 
tichals three, narrower than the costals; the two lower ones combined but 
little longer than the third, and united by syzygy, the second giving off a 
strong pinnule to the outer sides of the rays; the third axillary. Arms gener- 
ally four to the ray, occasionally five to six in one of the three anterior rays, 
or in all of them; they are widely divergent, comparatively thin, rather i 
short, and composed of a single series of cuneate pieces alternately arranged. 
Ventral disk depressed-convex, the median portions flattened and occupied 
by five, small but well defined orals, from which five rows of comparatively 
wide but short covering pieces pass out to the arms. The three interbrachial 
plates are followed by eight to ten interambulacral pieces, distributed in four | 
ranges. Anal plate a little narrower at the top than below, succeeded by | 
five small pieces transversely arranged, which in turn are followed by numer- | 
ous smaller plates forming a protuberance which contains the anal opening. | 
The anus is almost marginal and opens out laterally. Column small, round ; if 
the joints short, the nodal ones somewhat projecting. | 
Horizon and Locahty. — Upper Burlington limestone; Burlington, Iowa. 
Type in the (Worthen) Illinois State collection. 
Remarks. — This species is readily distinguished from any other of this ' 
genus by its peculiar style of ornamentation, in which, however, it closely re- | ; 
sembles Platycrinus subspinulosus Hall, to which we erroneousl y referred it in | 
Part II. of the Revision. The type specimen, which we had examined, was im- 
perfect, showing neither the anal plate nor the form of the calyx. Fine speci- 
mens since obtained by us in considerable number leave no doubt that it is an 
exceedingly well marked species, and was correctly described as a Dichocrinus. 
