530 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



(?) Periechocrinus Whitei (Hall). 

 PMe XLVI. Fig&. 1, ^, 3, and Plate LI. Figs. 9, 10. 



1861. Admoeriims {Megistocriniis) IF/iUei — B.A.LL; Prelim. Descr. New Criii., p. 2, aud Bost. Journ. Nat. 



Hist, p. 271. 

 1873. Megistoeriiws {Saecoerbms) jnUei — M^ES. and Wokthen ; Geol. Rep. Illiuois, Yol. V., p. 397, 



Plate 6, Figs, la, i, c. 

 1877. Megisloerimis W/diei — S. A. Millek; Araer. Palaioz. Poss., p. 83. 



1881. Periechocrimis »7/jfc! — W. aud Sp. ; Revision Palajocr., Part II,, p. 133; and Whitfield; Amer. 

 Mus. Nat. Hist., 1S93, Vol. I., p. 27, Plate 2, Pig. 29. 

 Syn. AHinocrims {Pradoerhms) ampins — Msek aud Wokthes ; 1861, Proc. Acad. Nat. Sei. Pliila., 



p. 133, also 1868, Geol. Rep. Illiuois, To). III., p. 470, Plate 16, Pig. 2. 

 Syn. JcHnoeritms amplus — S. A. Millek; Amer. Pakoz. Poss., p. 66. 

 Syn. Perieehocrinus amplus — W. and Sp. ; Revision Pateocr., Part II., p. 131. 



Calyx depressed bell-shaped, height and greatest width nearly equal; 

 tegmen flattened or depressed convex ; plates thin, and without ornamenta' 

 tion ; radial ridge but faintly indicated. 



Basals forming a shallow basin, abruptly truncated at the bottom for the 

 reception of the column ; the latter occupying two thirds the diameter of the 

 base. Radials large, length and width subequal, their upper lateral faces 

 shorter than the corresponding lower ones. First costals scarcely more 

 than half the size of the radials, proportionally a little longer, and hexan- 

 gular; second costals much smaller than the first, and pentangular. The 

 latter support four distichals, which according to the age of the specimen are 

 either all fixed or in part free. The distichals short, even the first which is 

 twice as long as the succeeding ones. Arm facets projecting, arranged in 

 pairs, the spaces between the rays twice as wide as those between their main 

 divisions. Arms long, gradually decreasing in size upwards, very delicate at 

 the top, biserial above the first to the third distichal. Pinnules long, closely 

 packed and rather stout ; their joints twice as long as wide. First inter- 

 brachial as large as the first costals or larger ; supporting two smaller j^Iates 

 in the second range, three in the third, and a number of irregular pieces 

 above. Anal interradius very wide; each one of the three plates resting upon 

 the first anal almost as large as the one plate of the regular sides. They are 

 followed by three smaller pieces, and these in large specimens by as much as 

 twenty or more plates, of which the upper row connects with the jolates of 

 the tegmen. The disk plates are small throughout, and closely resemble the 

 interambulacral plates of some Pentacrinid^e. Anus excentric, rising but 

 little above the general surface of the tegmen. Column stout, the joints 



