54 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



ends of the radial shields, which are very widely separated, can be 

 distinguished. The ventral surface of the disk is covered with small 

 imbricated plates, many of which carry club-spines more slender 

 than those of the dorsal surface, which always end in a few short 

 and unequal spinules. The genital slits are rather broad. 



The mouth shields, which are of moderate size, are lozenge- shaped, 

 broader than long, with an obtuse proximal angle bordered by two 

 straight sides, rounded lateral angles, and two distal sides which 

 are gently convex and which pass over into each other by a broadly 

 rounded angle. The adoral plates, which are of medium size, have 

 their long borders straight and almost parallel; they are, however, 

 a little broader outwardly than inwardly, and they give off a nar- 

 row process separating the mouth shield from the first side arm 

 plate. The oral plates are triangular and very high. The lateral 

 mouth papillae, three in number, are much elongated and cylin- 

 drical, with the point obtuse, and subequal; there is sometimes a 

 fourth supplementary papilla near the terminal papilla which is 

 shorter and thicker than the three others. The unpaired terminal 

 papilla is thick and short. The surface of these papillae is very 

 rugose. 



The upper arm plates are extremely small and occupy only a very 

 limited portion of the dorsal surface of the arms, the greater part of 

 which is covered by the side arm plates. These plates are triangu- 

 lar, broader than long, and widely separated, with a rather open 

 proximal angle and a very broad and only slightly convex distal 

 border. 



The first under arm plate is a little broader than long, quadran- 

 gular or trapezoidal, with the distal border narrow, the proximal 

 border broader and convex, and the sides divergent and straight. 

 The following plates are pentagonal, with an obtuse proximal angle, 

 the sides excavated by the tentacular pores, and a strongly convex 

 distal border. They are separated beyond the second; the latter is 

 very much broader than long, those following becoming as long as 

 broad, and finally a little longer than broad. 



The side arm plates, which are moderately projecting, carry at 

 the base of the arms ten spines, which, on the first brachial segments, 

 form on the dorsal surface a perfectly continuous series from one 

 side to the other; and on the following segments the two lateral 

 rows of spines still come very close together in the median linei. 

 The three or four first ventral spines are subequal, and their length 

 equals that of the segment; the length then increases up to the last 

 dorsal spine, which measures about three segments. These spines 

 are slender and transparent, and are provided with very strong, tri- 

 angular and pointed teeth, which are rather well spaced. 



