266 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



" Dorsal side possessing a rather large centro-dorsal plate, which 

 is surrounded by five plates occupying the position of radials, while 

 the other part is covered with numerous small disk plates. " 



Gfenoholotype and only species.- C. peculiaris Worthen and Miller. 



Gregory states that CJiolaster "appears to be allied to Aganaster, 

 but the structure of the ambulacral ossicles is unknown." 



CHOLASTER PECULIARIS Worthen and Miller. 



Cholaster peculiaris WORTHEN and MILLER, Geol. Surv. Illinois, vol. 7, 1883, 

 p. 329, pi. 31, figs. 4a, 4b. 



Original description. "The general form of the body is that of a 

 truncated pentagonal star, with a large circular disk. A centro- 

 dorsal plate is surrounded by five others, all of which are much larger 

 than other plates of the body, and occupy a centro-dorsal depres- 

 sion. The position of these five plates is that of radials, resting 

 upon a basal, and therefore all other plates of the body might be 

 regarded as radials and interradials, the latter being very numer- 

 ous, because of the wide separation of the rays at their junction 

 with the body. The interradials are polygonal, convex, or sub- 

 spinous plates, without much variation in size whether filling the 

 larger or smaller interradial spaces. The plates following the larger 

 radial plates, as above defined, in the direction of the rays, are not, 

 however, by their form or arrangement to be distinguished from 

 the interradials, though differing from those forming the rays. 

 The plates covering the dorsal side are thus divided into three 

 kinds, viz.: First, the larger plates filling the centro-dorsal depres- 

 sion; second, the disk plates which cover all other parts of the body 

 except the rays, and, third, the plates which cover the rays proper, 

 which in this species are easily distinguished from the central disk 

 plates. 



"The rays are short, abruptly truncated, and slightly expanded 

 at the apices by reason of an enlargement of the terminal plates. 

 They are widely separated from each other, though not at uniform 

 distances, and present the appearance of having been stuck on the 

 central disk, instead of having grown from it, an appearance more 

 marked, by reason of the change in the form of the plates, from the 

 disk to the rays. 



"One of the interradial spaces is much greater than the others, 

 so that a line may be drawn across the disk, leaving three entire 

 rays upon the smaller half. The back of each ray is covered by a 

 series of transversely elongated plates separated from the side 

 series by a longitudinally impressed line. A single series of plates 

 covers either side of a ray, interlocking with the transverse dorsal 

 series and directed backward toward the disk. 



"The oral plates are unknown, and no madreporiform tubercle 

 has been detected." 



