ig 
Hi | 
ss | 032 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
eu | it: 
vA | | 
ie brachial of the regular sides; succeeding rows generally containing five plates. 
Interdistichals one. Structure of tegmen and arms not known. 
) 7 | Horizon and Locality. — Lower Burlington limestone ; Burlington, Iowa. 
Lype in the Museum of Comparative Zovlogy. 
MEGISTOCRINUS O. and Sxuum. 
| Wi 1852. Owzn and Suumanp; U. S. Geol. Rep. Iowa, Wisc. and Minn., p. 594. 
ne | | 1858. Hat; Geol. Rep. Iowa, Vol. L., Part IL., p. 479. 
i | 1869. Mex and Wortunn; Proceed. Acad. Nat. Sci. Phila., p. 163. 
1873. Murx and Wortuen; Geol. Rep. Illinois, Vol. V., p. 393. 
| F | 1879. Zirren; Handb. der Paleont., Vol. I., p. 371. 
1 i | | 1881. W. and Sp.; Revision Palzocr., Part IT., p. 185 (Proc. Acad. Nat. Sci. Phila., p. 309). 
a 
Sees 
Fh, | 1890. §. A. Minter; North Amer. Geol. and Palewont., p. 260. 
\4 Mi | Specimens generally large; the calyx depressed, wider than high; 
w ij P| flattened on the bottom, and sometimes excavated; the plates heavy. ) 
i; ; | Basals three, closely anchylosed and not divisible, forming together a thick | 
_ Ml | hexangular plate, pierced by a large canal. Radials generally spread out | 
" | | horizontally, wider than long, and all hexagonal in outline. Costals of a 
i MW : similar form to the radials, and almost as large. The number of brachials 
ie i | participating in the calyx is quite variable among the species; in some of 
a | them the rays are free from above the distichals, while in others palmars, 
and exceptionally post-palmars, are incorporated. Arms biserial through- 
out, branching, and gradually diminishing in size upwards. Pinnules small " 
\ and rarely preserved. The food grooves of the arms covered by two rows 
of covering plates, bordered on each side by a series of well defined side 
pieces, which in some species enter the tegmen. Interbrachials numerous 
and in contact with the interambulacral pieces. Anal area very wide; the 
three plates of the first interbrachial row large, and followed by several 
ranges of from four to six pieces. Ventral disk low hemispherical, the 
orals and radial dome plates often isolated by small perisomic plates, which 
increase in number with the growth of the individual. Anus excentric, 
sometimes marginal. Column very large and long, with strong cirri at the 
distal end; the central canal wide and pentalobate. 
Distribution. — This genus appears in America in the Corniferous, survives 
the Hamilton and Kinderhook groups, and disappears before the close of the 
| Upper Burlington. In Europe it is probably represented by “ Actinoerinus ” 
i globosus Phillips, from the Mountain limestone of England. 
Type of the genus: Megistocrinus Evansi. 
