358 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The ventral surf ace of the disk between the radial shields and 

 the peripheral plate which I have noticed above is only occupied by 

 a transverse row of three very small plates, the median smaller than 

 the two others. The genital plate covers only a very restricted por- 

 tion of the ventral surface of the disk; it carries a few small and 

 low papillae which become elongated at the periphery of the disk. 

 The genital slits are very narrow, but very elongated, and they ex- 

 tend to the vicinity of the proximal angle of the mouth shields. 



The mouth shields are very large, elongated, at least half again 

 as long as broad, and pyriform, with a sharp proximal angle and 

 a very convex distal border. The adoral plates are very small and 

 narrow. The oral plates are more developed than those preceding 

 and broader, with the borders straight and parallel. The lateral 

 mouth papillae are generally only three in number; they are small, 

 low, and rectangular, the outermost very much broader than the 

 others. The unpaired terminal papilla is stouter than those on either 

 side, though it is not very much developed. 



The upper arm plates are lozenge-shaped and of medium size. The 

 first is a little broader than long with a very obtuse and rounded 

 proximal angle. The following are almost as broad as long or a 

 little broader than long, with the proximal angle sharper than the 

 distal angle, and the two proximal sides longer than the two others 

 which are united over a very obtuse angle. These plates are sepa- 

 rated from the second outward by a small interval which, in the distal 

 half of the arms, becomes almost as long as the plates themselves. 



The first underarm plate is triangular, almost as broad as long, 

 with the angles and the sides equal. The following are much de- 

 veloped, trapezoidal, with a very narrow proximal side and an ex- 

 tremely broad distal side ; the lateral borders are very divergent. At 

 some distance from the base of the arms these plates become pen- 

 tagonal, the proximal border being replaced by- two small sides, 

 making with each other a very obtuse angle ; the plates then become a 

 little longer than broad with a rather narrow and slightly convex 

 distal border. They are separated beyond the disk by an interval 

 which progressively elongates. These plates show throughout their 

 whole length a median depression in the form of a rather broad 

 groove with parallel borders, and the succession of these depressions, 

 supplemented by corresponding depressions on the side arm plates 

 when the ventral plates become separated, form a very clearly 

 marked furrow which is of considerable depth on the four or five 

 first plates; beyond this the depth gradually diminishes, but the 

 groove persists to the tip of the arms. This groove is only slightly 

 marked and not very deep on the first under arm plate, but it is 

 abruptly excavated on the second, on which it reaches its maximum 



depth. 



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