432 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Description. — The diameter of the disk does not exceed 2 mm. 

 The arms are 8 mm. in length; they are rather narrow at the base, 

 and their width diminishes very slowly; the arm segments are a 

 little broader than long. 



The disk is rounded ; the dorsal surface is a little swollen, and the 

 ventral surface is plane. The dorsal surface is covered with large 

 imbricated plates, which are not very numerous, among which may 

 be distinguished a central group of six plates somewhat irregularly 

 arranged and doubtless representing the primary rosette; the dorso- 

 central plate is ellipsoidal in form ; the five other plates, which are 

 slightly smaller, are rounded, but unequal. Outside of these plates 

 there may be recognized in the radial spaces a rather large triangular 

 single plate which extends between the two radial shields of each 

 pair, separating them in their proximal half. In the interradial 

 spaces there is a group of three consecutive plates; the first is large 

 and rounded, the second is rectangular and very short, and the third, 

 which is almost square, reaches the periphery of the disk. Here and 

 there a very small plate occupying the intervals between the large 

 plates may be met with, but these are very rare. The radial shields 

 are smaller than the plates of the central rosette ; they are triangular, 

 almost as long as broad, divergent inwardly and in contact outwardly 

 for a half or a third of their length. This length is a little greater 

 than one-third the radius of the disk. The surface of all these plates 

 is rather coarsely granulose. 



The ventral surface of the disk in the interradial spaces is occu- 

 pied by a small number of rounded, somewhat imbricated, and un- 

 equal plates. The genital slits are scarcely evident, and very narrow. 



The mouth shields, which are of medium size, are pentagonal, a 

 little longer than broad, with an almost straight proximal angle 

 bounded by two straight sides, and two straight lateral borders con- 

 verging toward a very narrow and rounded distal side; this distal 

 side is sometimes replaced by a rounded angle and the mouth shields 

 then assume a lozenge-shaped form. The adoral plates are rather 

 broad, almost twice as long as broad, with the long sides straight 

 and parallel. The oral plates are very small and triangular. The 

 mouth papillae number five on each side. The three outermost papil- 

 lae are rounded and squamiform; the proximal papilla is slightly 

 elongated, conical, with the point blunted; it is crowded against its 

 neighbor. 



The first upper arm plate is small, quadrangular or trapezoidal, 

 as long as broad. The second is rectangular, twice as broad as long, 

 with the sides divergent and the angles rounded. The following 

 plates are large, triangular, with a sharp proximal angle bounded 

 by two straight sides which pass over by rounded angles to the distal 

 border, which is itself slightly rounded. These plates are as long 



