52 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



straight sides, and these, passing around very rounded angles, unite 

 to form the distal border, which is almost straight. The adoral 

 plates are large and broad, tapering inwardly, though broadly in 

 contact in the median interradial line ; they are broadened outwardly 

 and separate the mouth shield from the first side arm plate. The 

 oral plates are triangular and high, and of medium size. The lat- 

 eral mouth papillae are sometimes three, sometimes four in num- 

 ber; they are small and conical with the point blunted; the un- 

 paired terminal papilla is a little larger than those adjacent. 



The upper arm plates are small and widely separated by the side 

 arm plates; they are triangular, broader than long, with an obtuse 

 proximal angle bounded by two straight sides which pass around by 

 sharp angles to the distal side, which is gently rounded. In the 

 terminal part of the arms these plates become a little longer than 

 broad. 



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

 distal border straight and narrow and the proximal border broader, 

 passing by rounded angles into the sides ; these are straight and con- 

 verge toward the distal border, which is narrow and straight. The 

 two or three following plates are triangular, broader than long, with 

 an obtuse proximal angle and a large and very convex distal border. 

 The second plate is just in contact with the first by its proximal 

 angle, and the following are separated by an interval which at first 

 is very short, but which becomes progressively greater. Toward the 

 fourth or fifth the plates elongate and at the same time their width 

 diminishes; they then become as long as broad, pentagonal, with a 

 proximal angle bounded by two straight sides, two lateral borders 

 slightly divergent and somewhat excavated by the tentacle pores, and 

 a rather narrow convex distal border. Toward the middle of the 

 arm these plates even become a little longer than broad, further on 

 assuming a simply triangular shape with a very sharp proximal angle, 

 and being as broad as long. 



The side arm plates are prominent and bear at the arm bases up 

 to ten narrow and slender glassy and strongly echinulate spines. 

 The length of these spines increases progressively from the first ven- 

 tral, which is a little shorter than the segment, to the eighth of which 

 the length is equal to at least three segments; the two other dorsal 

 spines have almost the same length as the eighth. The number of 

 these spines progressively decreases, falling little by little to six ; at 

 the same time the length of the dorsal spines diminishes and scarcely 

 exceeds that of the segment. 



The under arm plates show on their surface striations parallel to 

 their distal border which are very conspicuous ; similar striations are 

 seen on the side arm plates, but they become very much less evident 

 on the upper arm plates. 



