BATOCRINID^. 535 



tact and alternate with one another. Anus at the end of a proboscidiforni 

 protuberance, within the arm regions or a little above or below. Column 

 large and long. In a large specimen it was found preserved to a length 

 of three feet, but evidently was considerably longer ; it gradually increased 

 its diameter from 14 mm. at the top to 24 mm. at the distal end. Near the 

 calyx, the nodal joints, which at their edges are rounded and somewhat un- 

 dulating, project conspicuously over the youngest and those of intermediate 

 growth, but farther down on the stem the latter attain the size of the oldest, 

 and a wider joint always alternates with a narrower one. Toward the dis- 

 tal end all the joints become of the same length and width, their proximal 

 and distal surfaces are covered with fine radiating striae, and the lines of 

 union are zigzag. Central canal extremely large, occupying one third the 

 diameter of the joint, and obscurely pentalobate. The lower end of the 

 column has not been observed, but from detached parts, which undoubtedly 

 belong to this species, it appears that it was provided for quite a distance 

 with long and stout branches, which were perforated by a canal of elon- 

 gate form. 



Horizon and Lomlikj. — Found in both divisions of the Burlington group 

 up to the white crystalline layers of the upper bed ; Burlington, Iowa, and 

 Henderson Co., Ills. 



Remarks. — The size of this species is extremely variable. Among the 

 fifty-two specimens under examination, the calyx varies from 5 to 65 mm. 

 in length, and from 7 to 85 mm. in width. As a rule, the specimens from 

 the Upper Burlington division are a shade larger, yet White's type of 

 Ilecjistocrinus plenus, the Upper bed form, in the Museum of Comparative 

 Zoology, is considerably smaller than the largest specimens from the Lower 

 bed. M. pkmis was said to differ " by its convex base, prominent basal 

 plates, channeled sutures, and the proportions of the body plates ; " but 

 neither one of these distinctions will stand when a large number of speci- 

 mens are compared. The basals are not prominent in any of them— not 

 even in the type — and the suture lines are not more channeled than they 

 are in specimens from the Lower bed. Acfinocriims minor, A. mperlaius, and 

 A. hrevicornis, which we have always held to be one species, represent earlier 

 stages in the development of Megistocrinus Evansi, as we suggested in the 

 Revision (Part IL, p. 138). " Actinocrhms " minor with fewer anal plates, and 

 the anal aperture much lower on the posterior side, represents the younoest 

 form of tlie species, and A. superlatus and A. hrevicornis a transition between 



