478 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
1, 2, 2,2; the two upper on a level with the arm bases. Posterior area 
extremely wide, its median line rounded but not ridged. First anal plate 
followed by three large plates, and these by three smaller ones, which sup- 
port numerous others without definite arrangement. The plates of the 
tegmen are flat, except the posterior oral, which is slightly convex and 
central in position; the four other orals are pushed anteriorly, and are 
surrounded in large specimens by irregular perisomic plates. In smaller 
specimens, their outer edges abut against a large radial dome plate, from 
which two rows of covering pieces at both sides pass out to the arms. The 
anus 1s placed half way between the summit and the arm bases, and opens 
out laterally. Column small; composed near the calyx of wide and narrow 
joints. . 
Horizon and Locality. —Upper Burlington limestone ; Burlington, Iowa, 
and at the same horizon at several localities in Missouri and Illinois, 
fvemarks. — The arm formula of this species varies exceedingly. Among 
seventeen specimens there is one with #2 arms, five with 5: arms (this is 
3 
the arm formula of “ Actinocrinus” symmetricus Hall), six with 42 (as in 
2 
2 
§. A. Miller’s Dorycrinus amenus), one with %%, and four with 53 arms. As 
a rule, large specimens have more arms than smaller ones, and the specimens 
with ten arms are without exception small. The specimens are identical 
otherwise, and this proves most conclusively that the number of arms, in 
this group at least, cannot be made a specific character unless accompanied 
by other structural differences. Such, however, do not exist in Dorycrinus 
amenus Miller, nor in Actinocrinus symmetricus Hall. 
Aorocrinus spinosulus (Ha11). 
Plate XLITL. Fig. 4, and Plate XLIV. Figs. Sa, b. 
1859. <Actinocrinus spinosulus — Haui; Suppl. Geol. Rep. Iowa, p. 52. 
1881. Doryerinus spinosulus —W. and Sp.; Revision Paleocr. Part II., p. 180. 
Syn. Doryerinus Kelloggi — WortuEn ; Geol. Rep. Illinois, Vol. VI., p. 518, Plate 29, Fig. 8. 
A small species. Dorsal cup obconical, strongly lobed at the arm regions, 
truncated at the base; ventral disk almost flat and without spines. . Plates 
devoid of ornamentation, a little convex, the suture lines grooved. 
Basals large, forming a cylindrical cup, excavated at the bottom. Radials 
very large, longer than wide, the sloping upper faces short. First costals 
hexangular, of more than medium size, one third wider than long. Second 
Se SS 
4 
= 
OO 9 th 
