432 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
than the radials; quadrangular. Second costals a little larger; irregularly 
pentangular, Distichals in two rows, the upper wider and obtusely axillary. 
Palmars two, the plates of the upper row smallest, constituting quite fre- 
quently a part of the free arms. Arm openings placed at equal distances, 
except the two facing the anal interradius, which are somewhat farther apart. 
Arms normally twenty, but varying from eighteen to twenty-two; rather 
short, slightly flattened at the tips. Interbrachials from three to five, in two 
or three rows; those of the second row varying in height. The anal inter- 
radius generally has four ranges; 3, 3, 3, 2, arched over by the palmars. 
Ventral disk a little shorter than the dorsal cup, hemispherical ; the plates 
rather large and of nearly uniform size. Anal tube subcentral and com- 
paratively short. 
Horizon and Locahty.— Upper Burlington limestone ; Burlington, Iowa, 
Henderson Co., Ills., Palmyra, Mo., and at all localities where rocks of that 
formation are exposed, this species being one of its most abundant and 
characteristic fossils. 
Remarks. — Dizygocrinus rotundus is the most common species of the 
Burlington group; it occurs most abundantly in the white crystalline layers 
of the middle part. The number of arms is quite variable, we have speci- 
mens with 18, 19, 20, 21, and 22 arm openings. The arms, which are rarely 
preserved in this species, are simple, there being but one arm to each open- 
ing in all our specimens except one, which has a single paired arm in one of 
its rays. 
Hall's “ Actinocrmus” oblatus is a depressed Dizygocrinus rotundus with 
twenty-two arms. 
Dizygocrinus dodecadactylus (Mex and WortTHEy). 
Plate X XIX. Figs. La, 6, c. 
1861. <Actinocrinus dodecadactylus — Munk and W.; Proceed. Acad. Nat. Sci. Phila., p. 181. 
1866. <Actinocrinus (Batocrinus) dodecadactylus —M.and W.; Geol. Rep. Illinois, Vol. IL., p. 205, Plate 
15, Figs. 3a, 4, ¢. 
1873. Batocrinus dodecadactylus —MxEKk and WortHeEn; ibid., Vol. V., p. 368. 
1881. Batocrinus dodecadactylus — W. and Sp.; Revision Paleocr., Part II., p. 166. 
Probably an early phase of Dizygocrinus rotundus in a persistent form. 
It is a considerably smaller species, and has twelve instead of twenty arms. 
Calyx globular, the ventral disk as high as the dorsal cup. Plates smooth, 
suture lines moderately distinct. 
Base short, rounded, excavated at the bottom. Radials large, wider than 
long. Costals small, twice as wide as long, the upper one axillary, support- 
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