384 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The disk is pentagonal, with the sides rounded, and its form is 

 slightly oval. The dorsal surface is plane and the ventral slightly 

 convex ; the dorsal surface is strongly notched at the base of the arms. 

 It is covered with extremely numerous small plates, which are slightly 

 imbricated and of which the outlines are more or less obscured by a 

 thin integument. There may be distinguished a dorso-central plate 

 which is rounded and slightly larger than those adjacent, and, far 

 removed from it and almost equidistant from the center and from 

 the periphery of the disk, five primary radials scarcely larger than 

 the other plates, which, however, may be recognized by their slightly 

 darker color and their rounded form. All the other plates, which 

 are very small, are subequal ; but they become slightly larger in the 

 vicinity of the radial shields. These are small, irregularly tri- 

 angular, almost as long as broad, divergent, and separated outwardly 

 by a single row of plates ; their length is about equivalent to a sixth 

 or a seventh of the radius of the disk. The radial papillae are large, 

 elongated, slightly flattened, and rounded at the tip; they maintain 

 the same width over their whole length. 



The ventral surface of the disk is covered with numerous imbri- 

 cated and equal plates which are smaller than those on the dorsal 

 surface. The genital plates are narrow and scarcely evident; the 

 genital slits are elongated and narrow. The radial papillae which 

 I described above occur only on the dorsal side, and they disappear 

 on the ventral surface of the disk in such a way that the borders of 

 the genital slits are without papillae except at the proximal end, 

 where a few may be found which, however, are inconstant. 



The mouth shields, almost as long as broad, are pentagonal with 

 a rather open proximal angle ; the two sides are straight and notched 

 toward their proximal third by the bottom of the genital slits; they 

 pass over by very rounded angles into the distal border, which is 

 convex. The adoral plates are rather narrow and much elongated. 

 The oral plates are narrower than the adoral, and especially are 

 shorter. The lateral mouth papillae are four in number; the three 

 outermost are obtuse and rectangular ; and the last is more elongated 

 and pointed ; the unpaired terminal papilla is larger and conical. 



The arms are flattened. The three first upper arm plates, included 

 within the incisions of the disk, are small and short. The following 

 plates are very large, quadrangular, very much broader than long, 

 with the angles rounded and the sides convex; their width becomes 

 progressively reduced and they soon become as broad as long; they 

 are very broadly in contact. 



The first under arm plate is large, almost hexagonal, with a broad 

 proximal side and a narrower distal side which is notched in the 

 middle ; the sides are formed by the union of two small, straight bor- 



