404 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



plates; this plate does not occupy quite the entire width of the inter- 

 radial space. The radial shields are large, much elongated, triangular 

 in form, with the proximal angle very rounded; they are two and a 

 half times as long as broad, and they are not very much broader in 

 their distal portion than in their proximal. The internal borders of 

 the two shields of each pair are parallel and widely separated. Their 

 length is greater than half the radius of the disk. 



The ventral surface is covered with rounded plates which are 

 slightly imbricated, subequal, and larger than those which occupy 

 the central region of the dorsal surface. The genital plates are 

 narrow and elongated. The genital slits are very narrow and rather 

 short, occupying scarcely half the length of the interradial area of 

 the ventral surface ; they are provided along both borders with small 

 very low rectangular papillae with indistinct outlines. Beyond 

 the genital plate there is a small triangular space extending between 

 the region which separates the side arm plates of the second and 

 third arm segments and the border of the disk ; this space is covered 

 with small rounded granules, but these do not continue the genital 

 papillae from which they differ in shape. These small granules ex- 

 tend as far as the outer border of the radial shields, but they are not 

 visible on the dorsal surface; they are well seen only by viewing 

 the arm from the side (fig. 8). 



The mouth shields, which are of medium size, are triangular, 

 almost as long as broad, with a rather sharp proximal angle and 

 straight sides which pass over by rounded angles into the more or 

 less excavated distal side. The adoral plates are rather broad in 

 the outer two-thirds of their length, with the two longer sides 

 parallel; they then taper rapidly inward, not touching the median 

 interradial line except by a very short border. The oral plates are 

 triangular and rather high. The lateral papillae are six in number; 

 they are all rectangular and low, with the free border straight; the 

 second papilla is the largest, and the height decreases from the first 

 to the last, which still remains rectangular. The separation of these 

 papillae is not very marked. The unpaired terminal papilla is 

 short, triangular, and obtuse. 



The three first upper arm plates are extremely short, and rectangu- 

 lar in form. The following is still very broad, but triangular, with 

 a very obtuse and rounded distal angle. Beyond this the plates 

 assume a lozenge-shaped form with the two proximal sides longer 

 than the two others. The first of these lozenge-shaped plates has the 

 proximal angle truncated and is in contact with the preceding, but 

 beyond this the dorsal plates are separated by a space which in- 

 creases progressively. 



The first under arm plate is rather small, trapezoidal, with the 

 proximal border a little broader than the distal, which is slightly 



