J. 16 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The upper arm plates are rather large, triangular or slightly bell- 

 shaped, with a sharp proximal angle and a convex distal border. 

 From the base of the arm outward they are separated from each 

 other by a narrow interval. 



The first under arm plate is triangular, with a distal rounded 

 angle and the proximal side excavated; it is a little broader than 

 long. The second is very large, triangular, very much broader than 

 long, with a proximal angle, and with the distal border slightly 

 convex. The following plates become pentagonal, with a very ob- 

 tuse proximal angle and a convex distal border; they are at first 

 broader than long, but they become progressively elongated, and, 

 beyond the sixth, they are a little longer than broad. Beyond the 

 second they are separated by a narrow interval. 



The side arm plates, which project moderately, carry eight or 

 lane stout cylindrical spines with the point rounded and slightly 

 echinulated, especially on the ventral spines. The first ventral spine 

 is as long as the segment and the last dorsal equals two segments 

 and a half. 



The rather well-developed tentacle scale is always single. On the 

 pores of the first pair this scale is rounded, very broad, and rather 

 short; on the following segments it elongates a little and becomes 

 twice as long as broad, with the tip rounded. 



Affinities and distinctive features. — Like 0. sagittata, 0. subjecta 

 is very close to O. mutata Koehler, but it is distinguished from it by 

 the shape of the club spines on the dorsal surface of the disk, in which 

 the head forms three divergent lobes armed with very strong teeth ; 

 this head assumes a simply rounded form and gradually disappears 

 at the periphery of the disk in such a way that the club spines end 

 by resembling small true spines; and this last-mentioned character 

 they maintain over the whole extent of the dorsal surface in the 

 largest specimen from station 5325 ; but whatever may be the form 

 of the head, these club spines are sharply differentiated from those 

 which I have described in O. mutata, in which the head remains per- 

 fectly smooth. The mouth shields are shorter than in O. mutata, and 

 the proximal angle is more elongated and sharper; the upper arm 

 plates are larger, the spines are more or less strongly echinulated, etc. 



In O. sagittata the club spines of the dorsal surface of the disk are 

 similarly terminated by a trilobed head, the form of which is close 

 to that taken by the club spines in 0. suljecta toward the periphery 

 of the disk; 0. sagittata is further distinguished from 0. mutata and 

 from 0. subjecta by the form of the mouth shields, which is quite 

 remarkable. Furthermore, the mouth papillae also show peculiar 

 characters, the spines are somewhat echinulated, and the upper arm 

 plates are a little more elongated than in 0. mutata. 



