654 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



wide and rather deep notch. Facets occujiying half the width of the 

 plates, seini-circular, only excavated at their outer faces, the inner edges 

 of the plates being in a straight line, which is slightly notched for the 

 passage of the ambulacral vessels. Costals moderately large, sub-pentagonal; 

 their lateral margins very short, directed outward and slightly upward. Dis- 

 tichals and palmars once and a half as wide as long, the former giving off 

 an arm to one side, and two palmars to the other, the latter two arms, 

 making three arms to each subdivision, or six arms to the i-ay. Arms rather 

 slender, very slightly tapering, rounded on the back, their five or six prox- 

 imal plates wedge-form and uniserial, the succeeding ones biserial. The 

 arm plates are nearly as long as wide, and each one is marked by a small, 

 transverse elevation projecting from the sides of the arms. Pinnules nearl}', 

 or quite, in contact laterally. 



Ventral disk depressed hemispherical ; orals large, tuberculous, excen- 

 tric, and asymmetrically arranged ; disk ambulacra short and composed of 

 few rather large pieces. The interambulacrals consist of a single row of 

 three plates, of which the two at the sides are very narrow and bend 

 abruptly outward ; the middle one large, and resting invariably against the 

 orals. The middle plate of the anal side, which is larger and sub-quadrangu- 

 lar, is followed by two medium sized plates, and these by numerous smaller 

 ones, which together form an elongate protuberance or short anal tube, 5 to 

 6 mm. in length, directed upward, and curving slightly inward to the middle 

 of the disk ; its lower margin, bordering the posterior oral, slightly truncat- 

 ing its outer edge. Column moderately twisted, the joints decidedly elliptic 

 in the adult, almost circular in young specimens. 



Horisoii and Locality. — Lower Burlington limestone, Burlington, Iowa; 

 and found at the same horizon throughout Missouri, and in Southwestern 

 New Mexico. 



SemarJis. — The subglobose form of the calj'x, but still more the con- 

 struction of the ventral disk with its incurving anal tube and the large 

 interambulacral plates which invariably touch the orals, distinguish this 

 species from all others known to us. 



In describing this species we had for comparison over sixty specimens of 

 all sizes, from 12 mm., including arms, to 8 cm., showing the modifications 

 that took place in the growing Crinoid, in the form of the calyx and the 

 structure of the arms. The calyx of young specimens is shorter, the base 

 more depressed, the arms zigzag, and composed of long wedge-form plates, 



