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 Dorycriniis 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 sjnnosuhts 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. 



