300 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



eral equal to a third of the radius of the disk; but they are very 

 unequal. On two adjacent pairs the shields are widely separated 

 from each other by a row of plates; in the pair at the base of the 

 small arm the two shields are separated by two rows of plates, while 

 in the pair opposite the two shields are in contact. The two shields 

 of the fifth pair appear fused into a single plate which must have 

 united with a certain number of the neighboring plates, for it is very 

 much larger than the sum of two normal radial shields. 



The part of the ventral surface of the disk beyond the large mouth 

 shields and between the two genital plates is naked. The genital slits 

 are very broad. The genital plates are large, much broadened, three 

 times as long as broad. 



The mouth shields are extremely large, triangular, at least twice as 

 broad as long, with a very obtuse proximal angle ; the lateral angles 

 are broadly rounded, and the distal side is concave. The adoral plates 

 are large and short, half again as long as broad, and oval ; the oral 

 plates are large and high. The tooth papillae are arranged in three 

 rows; the external papillae are larger and stouter than those in the 

 median region. The mouth shields and the adoral plates are covered 

 with small shining granules, surrounded by a dark circle, and widely 

 separated from each other ; the same granules occur on the under arm 

 plates. 



The first upper arm plates which follow the dorsal plates separat- 

 ing the radial shields of each pair, are at first small and more or less 

 irregular, or even broken up into unequal fragments, and they do not 

 cover the whole space outwardly bounded by the side arm plates; 

 but these plates soon become regular and then appear very large, 

 twice as broad as long, with the long sides parallel, the distal a little 

 convex, and the proximal a little concave ; the sides are each bounded 

 by two small straight borders forming between them an obtuse angle. 

 At some distance from the base of the arms, these two sides are re- 

 placed by an uninterrupted border directed obliquely outward and 

 united by a sharp angle to the distal side, this last from that point 

 onward becoming appreciably larger than the proximal side. All 

 these plates are in contact. 



The first under arm plate is quadrangular, a little longer than 

 broad. The second is also quadrangular, with a very short and 

 rounded proximal border, the sides strongly divergent and excavated 

 in the middle by the corresponding tentacle pore, and a very 'broad 

 and very strongly convex distal border; this plate is a little longer 

 than broad. On the following plates the proximal border broadens 

 and becomes equal to the distal side, while the sides become very 

 much shorter; the plates then assume a strictly quadrangular form. 

 The third plate is almost as long as broad, but the following become 



