~~ 
BATOCRINID &. 479 
costals pentagonal, a little larger than the first, their sloping upper faces in 
all the rays supporting an axillary distichal, and this two small palmars, 
which among themselves, and with the two of the opposite side of the ray, 
are in contact laterally. Arm openings small, four to the ray; arms simple, 
very short and delicate. Interradial spaces wide and deeply depressed 
between the free rays. Regular interbrachials 1, 2, 3, decreasing in size 
upward; the first not larger than the first costals; the two of the second 
row somewhat smaller; those of the third narrow, occupying the depressions 
between the rays. The interbrachials are followed by three or four minute 
interambulacral pieces which meet the orals. Anal interradius extremely 
wide, the interspace at the arm bases twice that of the other rays. There is- 
a vertical row of four anal plates, followed by a number of irregular pieces, 
which form an almost flat area and enclose the anal opening. At each side 
of the second anal plate there is a good sized interbrachial, and to each side 
of the second are two smaller pieces. Orals large, occupying fully one half 
‘the surface of the disk; they are convex, sometimes conical but not spin- 
ous; the posterior one is a little larger than the four others and pushed in 
between them, but, as a rule, the orals of this species are more symmetri- 
cally arranged and proportionally larger than usual in this genus. The 
primary radial dome plates resemble the orals in form and size, and occupy 
the outer end of the lobes. Anal opening directed laterally, and placed 
almost on a level with the arm bases. 
Horizon and Locality. — Keokuk group, Keokuk, Iowa, and Nauvoo, Ills. 
Type in the (Worthen) Illinois State collection, Springfield. 
Remarks. — We regard Dorycrinus Kelloggi Worthen as identical with this 
species, from which it was said to differ in the number of arms. That species 
was described as having but three arms in one of the antero-lateral rays, 
a structure which clearly indicates an irregular development of the rays. 
Aorocrinus spinosulus is closely related to A. parvus Shum. from the 
Burlington group, and its structure indicates that it is a descendant of 
that species, but sufficiently differentiated to be ranked as a good variety, 
if not as a full species. 
