378 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The disk is rounded. The dorsal surface is depressed in the central 

 region, which is entirely without plates and covered only by a very 

 thin membrane of a light yellowish brown color in the larger speci- 

 men, greenish in the smaller. The uncalcified region is relatively 

 larger in the second specimen, and from it extend ten prolongations ; 

 five radial and five interradial, which run toward the periphery of the 

 disk, though without reaching it, and give to this membranous part 

 a stellate form. This feature is nothing out of the ordinary, and it is 

 observable also in specimens of 0. flagellata of medium size (pi. 85, 

 fig. 6). The remainder of the dorsal surface is covered with very 

 small, thin, transparent, imbricated plates, somewhat unequal in size, 

 which become stouter toward the periphery of the disk and in the 

 vicinity of the radial shields. These last are very small, triangular, 

 longer than broad in the small specimen, almost as long as broad in 

 the larger, in which they are relatively less developed. The papillae of 

 the radial comb are very large, elongated, rather thick, conical, and 

 pointed. 



The ventral surface of the disk in the interradial spaces is covered 

 with plates larger than those of the dorsal surface. These plates are 

 smaller toward the periphery of the disk, where they are imbricated, 

 and they become very much larger in the vicinity of the radial shields, 

 where the imbrication ceases. The genital slits are narrow. The 

 genital plates are narrow and elongated; they show on their free 

 border some papillae which are at first small, short, and pointed, 

 but which rapidly become elongated toward the periphery of the 

 disk. 



The mouth shields are large, triangular, with a rather open proxi- 

 mal angle bordered by straight sides passing over by very rounded 

 angles into the broad and straight distal border. The form is almost 

 the same in the two specimens ; though in the larger the shields are a 

 little broader than long, and in the smaller they are as long as broad. 

 There is not the least trace of notching at the level of the bottom of the 

 genital slits. The adoral plates are narrow and extremely elongated, 

 narrowed slightly a little before their ends by the mouth tentacle pore, 

 and they are in contact in the median interradial line. The oral plates 

 are a little broader and very much shorter than the last. The mouth 

 papillae number six or seven on each side ; the outermost papillae is 

 very short, low, and scarcely evident, the length then progressively 

 increasing to the last, which is elongated, conical, and a little smaller 

 than the terminal papilla with which it is in contact. 



The three first upper arm plates, included within the notching 

 of the disk, are small and narrow; their length increases from the 

 first to the third ; their form also varies. The third plate is already 

 two or three times as broad as long, the width still further increasing 

 on the two following plates, and the fifth being almost twice as broad 



