536 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
the young and the adult. The two latter are said to differ in the number of 
“interbrachials, in their surface markings, and the condition of the posterior 
oral, whether spinous or not, — characters unreliable for specific separation. 
Megistocrinus parviwostris M. and W., which is comparatively small, and of 
which the anus is described as located below the arm bases, is the younger 
stage of “ Megistocrinus plenus,’ both coming from the Upper bed. The 
position of the anus in this species is quite variable, being sometimes be- 
low the arm regions not only in the smaller specimens, but frequently also 
in the larger ones. | 
This species is interesting for the light it throws upon the growth of 
the individual. In the smallest, and, as we think, youngest specimens, 
there are but two arm openings, the brachials being free above the second 
distichals, and remaining to a large extent in the condition of arm plates. 
The larger and more mature specimens, with the interbrachials increased to 
more than twice their previous number, have four arm openings, and the 
biserial distichals and palmars, the latter as far as to the fifth row, form part 
of the calyx walls. The tegmen in its earlier form is composed of but few 
plates, and the orals are in contact among themselves and with the radial 
dome pieces. Gradually with growth small supplementary pieces were intro- 
duced, and between these, which increased in size, were interposed in turn 
hundreds, and even thousands, of minute secondary pieces, still more increas- 
ing the capacity of the calyx, and encroaching upon the ambulacra so as to 
leave but a few isolated ambulacral plates exposed at the surface. 
' Megistocrinus Evansi var. crassus (Wurt®). 
1862. Wuitz; Proceed. Bost. Soc. Nat. Hist., Vol. IX., p. 17. 
1881. W. and Sp.; Revision Paleocr., Part IT., p. 137. 
Calyx very large, differing from J. Hvansi in the massiveness of its 
plates and the greater depression of the base. The radials are formed into 
high, broad knobs with deep channels along the interradial and _basi-radial 
sutures, and their surfaces are covered with coarse irregular corrugations. 
The nodes of the brachials and interbrachials are more conical. 
Horizon and Locahty. — Lower Burlington limestone ; Burlington, Iowa. 
Type in the Museum of Comparative Zovdlogy. 
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