Carboniferous Rocks of the Mississippi Valley. 309 



acutely angular above, supporting on each upper margin 

 the arm plates. Anal plates undetermined. Arms strong, 

 simple from their origin on the second radial plate, very- 

 long in proportion to the size of the body, angular on the 

 back, composed of a single range of plates alternately 

 longer and shorter on opposite sides ; each plate is fur- 

 nished, on the longer side, with a strong process projecting 

 laterally for the attachment of the tentacula, giving a zig- 

 zag appearance to the arms ; tentacula long, composed of 

 strong plates. Surface of calyx plates granulose or striato- 

 granulose. 



This species differs fi-om any other described in the pe- 

 culiar structure of the simple arms ; the projections for the 

 attachment of the tentacula give an oblique direction to 

 the plates, and this feature alternating in successive plates 

 gives an apparent spiral form to the arm. The tentacula 

 are comparatively strong, the lower joints a little longer 

 than wide, and nearly as long as the arm joints. 



Geological formation and locality. Burlington lime- 

 stone, Burlington, Iowa. Collection of B. J. Hall, Bur- 

 lington. 



Scaphiocrinus tortuosus (n. s.). General form and pro- 

 portions of body similar to the preceding, the basal plates 

 being somewhat larger, the sub-radials and first radials are 

 of nearly the same form and proportions ; second radials a 

 little shorter and broader in the upper part, single in the 

 antero-lateral and postero-lateral rays, while in the ante- 

 rior ray there are two plates in this position, the length of 

 both but little greater than the one in the other rays, and 

 of less width. 



Arms simple throughout their entire length ; the plates 

 short, concave on the short side and strongly angular on 

 the longer side from the projecting bases for the attach- 

 ment of the tentacula. Surface of calyx plates marked 

 by a series of ridges radiating from the centres of the 



