Carboniferous Rocks of the Mississippi Valley. 315 



ScapMocrinus rohustus (n. s.). Body calyculate, mam- 

 millEeform, rounded at base, moderately expanding from 

 the top of the basal plates. Basal plates pentangular, 

 pointed above, Jess than one half their length covered by 

 the column. Sub-radial plates of moderate size, four 

 equal and hexagonal, one larger and heptagonal, all wider 

 than high. First radial plates larger than the sub-radials, 

 scarcely equalling them in height, width nearly twice as 

 great as the height, four pentagonal, the one in the left 

 postero-lateral ray hexagonal. Second radials nearly as 

 large as the first, a little longer in their extreme height, 

 sub-pentagonal, angular above, and supporting on each 

 upper side a simple, strong arm ; the first arm plate much 

 larger than the succeeding ones, longer on the outer than on 

 the inner sides, which are closely joined. The arm plates 

 above are very short, strong, slightly wedge-form, very 

 gradually decreasing in length from below upwards. At 

 an inch above the second radial, the width of the arm is 

 more than twice the length of the arm plates on their long- 

 est sides. The first anal plate is nearly twice the size of 

 the succeeding ones, and somewhat smaller than the small- 

 est sub-radial plate, elongate hexagonal, placed obliquely 

 on the upper sloping edges of two sub-radials, and sup- 

 porting on its right upper side the lower edge of a first 

 radial plate, and on the remaining upper sides the two 

 succeeding anal plates, which are all that can be deter- 

 mined of the series. Surface of plates smooth ; cicatrix 

 of column attachment large, circular. 



In some individuals the arms become thickened in the 

 middle and contracted above, giving an elongate, fusiform 

 character. 



This species resembles Cyathocrinus {Poteriocrinus) 

 decadactylus of Lyon and Casseday, but diflfers in having 

 only two radial plates in each series instead of three, and 

 also differs in the anal series as described by those au- 



