618 GEOLOGICAL SURVEY OF IOWA. 



tremity sustains in part two plates, which rest mainly on 

 the outer edges of the brachial plates, and, curving upwards, 

 rise to the level of the top of the arm-bases. 



First anal plate much elongated, sustaining on its upper 

 truncated edge a large plate, and on its lateral oblique sides 

 partially supporting two large plates which rest mainly on 

 the adjacent radial plates, and extend upwards adjoining the 

 first and second brachial plates, and reach nearly to the top 

 of the arm-bases. The anal plates above these are somewhat 

 obscure, but apparently consist of about four series ; the 

 central portion protruding in a rounded ridge, with a broad 

 depression on either side : aperture opening upwards. 



On the third plate above the last radial, the arm-plates 

 subdivide, giving origin to a pair of strong arms, each pair 

 of which are separated at base by the thickness of two bra- 

 chial plates, and sometimes an axillary plate. 



The arms consist of a double series of short but very wide 

 plates, which are abruptly wedgeform at their junction with 

 each other. 



DOME composed of numerous strong tuberculiform plates, 

 a series of more prominent ones extending from the arms 



FIG. 96. 



The accompanying diagram illustrates 

 \ ^~ = ~2^~ v the form and proportions of the plates 



I AX^ /~~\ n^\ \\ 



>/ ( \ \ y / I |\v\\\^ * this species, as seen in the specimen 



figured on Plate xvi, fig. 2. 



Figs. 1 and 2 (of diagram) are trans- 

 verse sections of the arms ; fig. 1 re- 

 presenting a section near the base where 

 the outer edge of the plate is compressed 

 by the side of the adjacent arm of the 

 same ray, while fig. 2 represents a sec- 

 tion of the arm at a higher point. 





S 



