Carboniferous Rocks of the Mississippi Valley. 271 



first, plate eight-sided, supporting two narrow, elongate 

 plates, which occupy the spaces between the arnns of the 

 adjacent rays. First anal plate seven-sided, nearly or 

 quite as large as the first radial plates, supporting three in 

 the second range, with smaller plates above (the arrange- 

 ment of which is not determined) ; these extend up to the 

 aperture of the somewhat protruding anal area. Surface 

 of body j)lates smooth, with ridges in the direction of the 

 rays, caused by the elevation of the centres of the plates 

 of the radial series. Dome composed of numerous small, 

 slightly convex, polygonal plates. 



The peculiarity of this species consists in the anal open- 

 ing, the central spine, the protruding arm bases, and the 

 ridges traversing the radial series. It resembles the species 

 of the type of A. cornig'erus ; but ditfers in the absence of 

 lateral spines or tubercles over the centre of each ray, as 

 well as by the presence of the ridges. It differs also from 

 A. levcosia in the arm formula and in the ridges, but like 

 that has no lateral spines. The anterior ray is slightly im- 

 perfect in the individual described, but the form and size 

 of the lobe leave no doubt in regard to the number of 

 arms. 



Geological formation and locality. Burlington lime- 

 stone, Burlington, Iowa. Collection of C. A. White. 



Actinocrinus (Megistocrinus) Wiiitei (n. s.). Body po- 

 cilliform, the height and greatest breadth nearly equal ; 

 dome flattened or depressed-convex. Basal plates of me- 

 dium size, spreading almost horizontally from the top of 

 the column. First radial plates moderately large, height 

 and width sub-equal, superior lateral sides nearly equal to 

 the lower. Second radial plates proportionally large, hex- 

 agonal. Third radial plates pentagonal, sometimes hexag- 

 onal, smaller than the second radials, wedge-form above, 

 and supporting on each upper side a short hexagonal plate, 

 upon which rest the first arm plates. 



