536 THE CEINOIDEA 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 parvirostris 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 " Ilegistocrinus 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 

 hiserial 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 piefies 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 (White). 



1862. White ; Proceed. Bost. Soc. Nat. Hist., Vol. IX., p. 17- 

 1881. W. and Sp. ; Revision Palseocr., Part II., p. 137. 



Calyx very large, differing from M. Evansi 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 Locality. — Lower Burlington limestone ; Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



