PALEONTOLOGY OF IOWA. 563 



brachial plates. First anal plate resting against the column, 

 hexagonal, truncated above and supporting on its narrow 

 summit an elongate subspatulate heptagonal plate which 

 widens rapidly above, and upon its upper lateral faces two 

 smaller elongated plates : those, on the outer side, lie against 

 the upper radial and the brachial plate. The central upper 

 anal plate supports in part two succeeding plates in the same 

 series, nnd each of the lateral second anals supports a nar- 

 row plate lying between the arms and the central anal plate 

 of the second series. From the summit of the first arm-plates 

 to the anal opening there are three distinct arched series of 

 small plates, with two others centrally placed under the 

 crown of the two lower arches. 



The dome is composed of a series of very prominent tu- 

 berculiform plates converging from the arms, and other 

 smaller intervening plates in two or three ranges, with a 

 very prominent central tubercle. 



At the base, and within each pair of arms, is a small 

 pentagonal plate, resting the two lower sides against the 

 lower ranges of arm-plates, and supporting on its upper side 

 a large prominent tubercle. Above this, and arranged some- 

 what on each side are two small plates supporting a second 

 tuberculiform plate, which, with other similar plates, sur- 

 rounds the apicial tubercle, except on the anal side, where 

 there are several smaller plates. These rows of tuberculife- 

 rous plates divide the crown into five spaces occupied by 

 smaller plates, four of which are equal to each other, while 

 that on the anal side is much larger. 



This species differs sufficiently from dgaricomnus tuberosus to be readily distin- 

 guished. The base is more abruptly and deeply depressed, while the arms spread less 

 horizontally. The number of plates on the anal side is less numerous, and the range 

 of tuberculiferous plates between the base of the -arms and the summit is two instead 

 of three. 



Fig. 11 a. View of the anal side, seen a little from above. 

 Fig. 11 b. Base of same specimen. 



