414 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



12° 12' 35" N., long. 124° 02' 48" E. ; 247 meters (135 fathoms) ; 

 March 13, 1909. 



One specimen (Cat. No. 40993, U.S.N.M.). 



Description. — The diameter of the disk is 14 mm. ; the arms are 

 all broken off at some distance from their base. 



The disk is rounded ; the dorsal surface is slightly convex, and the 

 ventral surface is quite plane. The arms are rather broad, and they 

 taper gradually and regularly; the ventral surface is plane; the 

 dorsal surface is slightly depressed along the median line. 



The dorsal surface of the disk is covered with plates which are not 

 very numerous and rather large, and the radial shields are very 

 large; the general appearance strongly recalls the arrangement de- 

 scribed and figured by Lyman in O. simplex. There can be distin- 

 guished a primary rosette of contiguous plates, the polygonal dorso- 

 central and the radials, which are almost as large, and slightly 

 broadened transversely. Then comes a pair of smaller plates, and 

 then a series of three plates separating the two radial shields of each 

 pair; the first of these plates is large and triangular, the second is 

 small, elongated and very narrow, and the third is short and trape- 

 zoidal. In the interradial spaces there are found two successive large 

 plates which are longer than broad, the outermost of which lies very 

 near the periphery of the disk and is bounded below by two small 

 rounded and projecting plates ; some very small plates are seen here 

 and there between the preceding, but they are not numerous. The 

 radial shields are very large and very broad, triangular, with the 

 angles rounded; they are half again as long as broad, and their 

 length equals or slightly exceeds half the radius of the disk ; the two 

 shields of each pair are slightly divergent inwardly, and are sepa- 

 rated for their whole length. 



The ventral surface of the disk beyond the large mouth shields is 

 occupied by a single large plate which is quadrangular, with the sides 

 divergent and rounded, and a convex distal border which passes 

 over into the sides by very rounded angles. On either side of this 

 large plate is found the small projecting plate described above, 

 which is visible both from the dorsal and from the ventral surface. 

 The genital plate is much broadened and is notched toward its 

 proximal third by the genital slit. This, of a very peculiar form 

 and very short, appears as a small triangle the apex of which notches 

 the genital plate, and the base of which rests upon the first side 

 arm plate toward its distal angle. 



The mouth shields are large, elongated, and triangular, with th>> 

 proximal apex sharp and produced, extending between the adoral 

 plates ; the sides are slightly convex, and the distal border is straight 

 or slightly excavated ; they are very much longer than broad. On 

 four of them the proximal region is separated from the rest by a 



