PALEONTOLOGY OF IOWA. 595 



plates, and supporting the arm-joints. Dome composed of 

 numerous subangularly tuberculous plates, and the proboscis 

 of similar plates. 



SURFACE granulose or granulose-striate, with a plain slight- 

 ly elevated band along the margins of the larger plates. 



This species resembles in some degree the A. cequalis ; but it is more depressed, 

 with a smaller number of arms and a single series of interradial plates. The discoid 

 form is characteristic. 



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

 lington, Iowa. 



Actmocriiras ventricosus ( n. s.). 



PLATE XI. FIG. 6 a, b. 



CALYX subturbinate, ventricose above, the anal side much 

 shorter : dome very convex, with a depression in the centre 

 for the aperture, the sides deeply channelled for the divisions 

 of the arm-plates ; base concave for the attachment of the 

 column. Basal plates short ; lower margins angular. Anterior 

 and antero-lateral first radial plates much wider than long ; 

 postero-lateral radials as wide as long. Second radial plates 

 hexagonal (the right postero-lateral one heptagonal). Third 

 radials heptagonal, and supporting on each of their upper 

 oblique sides a heptagonal supraradial, which in turn supports 

 two others, the outer of which are brachial plates ; and the 

 adjacent inner ones each support two plates, the inner of 

 which are brachial plates, and the outer ones of the two 

 being pentagonal and supporting brachial plates ; making 

 eight arms from each ray, all the rays being apparently alike 

 (with one exception where one ray has seven and another 

 nine arms). 



First interradial plate hexagonal, the size of the second 

 radial ; to this succeed two series of two each, nearly equal 

 in size, and a fourth series of two much smaller plates which 

 support the outer second supraradial plates of two adjacent 

 rays. The supraradial plates likewise enclose an area of two 



