532 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



brachial of the regular sides; succeeding rows generally containing five plates. 

 Interdistichals one. Structure of tegmen and arms not known. 



Horizon and Locality. — Lower Burlington limestone; Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



MEGISTOCRINUS O. and Shum. 



1852. Owen and Shumakd; U. S. Geol. Rep. Iowa, Wise, and Minn., p. 594. 



1858. Hail; Geol. Rep. Iowa, Vol. I., Part II., p. 479. 



1869. Meek and Wortuen ; Proceed. Aead. Nat. Sci. Phila., p. 163. 



1873. Meek and WoRinEN ; Geol. Rep. Illinois, Vol. V., p. 393. 



1879. ZiTTEL ; Handb. der Pala;.out., Vol. I., p. 371. 



1881. W. and Sp. ; Revision Palajocr., Part II., p. 135 (Proc. Acad. Nat. Sci. PHla., p. 309). 



1890. S. A. Miller; North Amer. Geol. and Palasont., p. 260. 



Specimens generally large ; the calyx depressed, wider than high ; 

 flattened on the bottom, and sometimes excavated ; the plates heavy. 

 Basals three, closely anchylosed and not divisible, forming together a thick 

 hexangular plate, pierced by a large canal. Eadials generally spread out 

 horizontally, wider than long, and all hexagonal in outline. Costals of a 

 similar form to the radials, and almost as large. The number of brachials 

 participating in the calyx is quite variable among the species ; in some of 

 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. 



Bistribidion. — 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 " Aciinocrinus " 

 rjlohosus Phillips, from the Mountain limestone of England. 



Type of the genus : Alegistocrinus Evansi. 



