58 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



angular, a little longer than broad, with the angles and the borders 

 rounded. Each of these lies at the end of a small projecting rib 

 the length of which is almost equal to half the radius of the disk. 

 The two shields of each pair are very broadly separated by a space 

 which is almost equal to the width of the arms. 



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

 with small plates which are rounded or slightly imbricated, ex- 

 tremely thin, and transparent, and which are not always perfectly 

 in contact. The genital slits are much elongated and rather narrow. 



The mouth shields are small and triangular, as broad as long, or a 

 little longer than broad, with a very sharp proximal angle bordered 

 by two straight or slightly concave sides, and a convex distal border 

 with a very slightly marked small lobe in the middle. The adoral 

 plates, which are of medium size, rapidly taper inwardly and are 

 broadened in their external portion, which gives off a rather narrow 

 process separating the mouth shield from the first side arm plate; 

 they are about two and a half times as long as broad, and their longer- 

 edges are slightly incurved. The oral plates are triangular and 

 rather high. The lateral mouth papillae are three in number ; the two 

 outer are cylindrical, rather elongated, with the tip rounded; the 

 internal is a little stouter, conical, with the point blunted; the un- 

 paired terminal papilla is a little thicker than those on either side. 

 The surface of these papillae is very rugose. Beyond the external 

 papilla there is in addition a smaller papilla which is inserted at the 

 angle between the adoral plate and the first under arm plate. 



The rather small upper arm plates are triangular, broader than 

 long, with a rather open proximal angle bordered by two straight 

 sides and a slightly convex distal border. They are separated out- 

 ward from the base of the arms by a narrow interval which does not 

 increase much distally. 



The first under arm plate is rather broad, triangular, broader than 

 long, with the distal border convex and the proximal angle obtuse. 

 The second, already separated from the first, is very broad and short, 

 triangular in form, with an obtuse proximal angle, sharp lateral 

 angles, and a convex distal border. The following plates become 

 pentagonal with a very obtuse proximal angle, the sides notched by 

 the corresponding tentacle pore, and a very convex distal border. 

 These plates are at first very much broader than long, becoming a 

 little longer than broad in the outer half of the arms. They are 

 always separated by a rather long space occupied by the side arm 

 plates. 



The side arm plates carry eight strong, thick and slightly flattened 

 arm spines. The ventral spines are especially flattened, and they 

 bear rather strong denticulations, which on the three first are par- 



