PALEONTOLOGY OF IOWA. 565 



Actinocriiras pyramidatus ( n. s.). 



BODY depressed pyramidal, with the apex terminating in 

 a short acute spine. Basal plates not visible beyond the cir- 

 cumference of the column. First radial plates much longer 

 than wide, deeply concave towards the column, the outer 

 margins thickened and prominent, and, together with the 

 thickened second radials, forming the plane of rest where 

 the body is deprived of the column. Second radials short, 

 quadrangular. Third radials pentagonal, each one supporting 

 on its upper sloping sides two succeeding simple brachial 

 plates, which are in turn succeeded by a double series of 

 arm-plates, making one pair of arms at the base from each 

 radial. Interradial plates in general form ovate, resting on 

 the upper sloping edges of the adjacent radials : each one 

 is supported on its sides by the second and third radial, and 

 on it supper slopes by the two simple brachial plates ; its 

 upper short side supports a narrow second interradial or in- 

 terbrachial plate. First anal plate much longer than the first 

 radial plates : second, third and fourth ranges consisting of 

 three plates each ; and above this, two or three ranges of 

 small plates surrounding the aperture which lies just below 

 the base of the apicial plate. The centre of the entire anal 

 area, from the first plate to the apex, is elevated in a dis- 

 tinct rounded ridge. 



Summit short pyramidal : a range of two or three promi- 

 nent tuberculous plates extends from the upper side of the 

 arms to the apicial plate ; those from the anterior pair of 

 arms being surmounted by two plates, making six tuberculi- 

 form plates surrounding the apicial spiniferous plate. Arms 

 two at their origin from each ray. Surface somewhat coarsely 

 granuliferous. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



