478 THE CRINOIDEA CAMEEATA 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 sujd- 

 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 perisoniic 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 is placed lialf 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 LocalUy. — Upper Burlington limestone; Burlington, Iowa, 

 and at the same horizon at several localities in Missouri and Illinois. 



BemarJcs. — The arm formula of this species varies exceedingly. Among 

 seventeen specimens there is one with i^ arms, five with iji arms (this is 

 the arm formula of " Actinocrinus" symmetricits Hall), six with ijl (as in 

 S. A. Miller's Dori/crinus amoeniis), one with i^, and four with iji 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 

 amoBnus Miller, nor in Actinocriniis symmetricus Hall. 



Aoroerintis spinosulus (Hall). 

 Plate XLIIL Fig. 4, and Plate XLIV. Figs. 8a, h. 



1859. Aciinocrinus spinosnlus — Hall; Suppl. Geol. Rep. Iowa, p. 53. 



1S81. Dorycyiiins spinosulus — ■ W. and Sp. ; Revision Palreocr. Part II. j p. ISO. 



Sjn. Dori/crims Kelloggi — Woethen ; Geol. Rep. Illinois, Vol. VI., p. 513, Plate 39, Pig. 8. 



A small species. Dorsal cup obconical, strongly lobed at the arm regions, 

 truncated at tlie 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. Eadials 

 vei-y large, longer than wide, the sloping upper faces short. First costals 

 hexangular, of more than medium size, one third wider than long. Second 



