28 THE VOYAGE OF H.M.S. CHALLENGER. 



Ophiothyreus. 

 Ophiothyreus, Ljn., Of. Kong. AkacL, 1871. 



Disk and arms enclosed by stout, swollen plates. Large tentacle pores quite to the end 

 of the arm, closed by thick scales. Arm spines few and minute. A row of papillae passes 

 along the outer edges of the genital scales and of the radial shields; between the latter is 

 wedged a pair of plates (a divided upper arm plate), along whose inner edges runs a 

 corresponding row of fine papillae. Two genital openings in each interbrachial space. 

 Although small as seen from above, the radial shields from the inside are found to be 

 so large, as to form a closed ring round the disk margin. 



The genital plate is short and stout with an extremely wide clubbed head, to which is 

 joined the short, flat, very thick genital scale. Peristomial plate large, and composed of 

 two closely soldered halves. The arm bones are pretty stout, the first two being discoid, 

 and not grooved on the edges, while those beyond are much longer on top. 



Species of Ophiothyreus not herein described. 



Ophiothyreus goesi, Ljn., Dr Goes, Oph. Of. Kong. Akad., p. 619, 1871 ; Lym., Bull. 

 Mus. Comp. Zool., vol. v., part 9, p. 222. 

 "West Indies ; 80 to 300 fathoms. 



Ophioplinthus. 

 Ophioplinthus, Lym., Bull. Mus. Comp. Zool., voL v., part 7, 1878. 



Disk smooth and covered by a thin skin bearing irregular delicate scales and radial 

 shields. Genital scales wide and divided in several pieces. Small, blunt, close-set 

 mouth papillae; no tooth papillae; short angular teeth; very minute, peg-like arm spines 

 on outer edges of side arm plates. Second pair of mouth tentacles and first two pairs of 

 arm tentacles rising from round pores near the inner end of the under arm plates; those 

 beyond are smaller, and stand close to the under arm spine. Arms narrow, cylindrical, 

 and gradually tapering. Two genital openings, running only a part of the way toward 

 the margin. Mouth frames seen from above, after removing" the top of the disk, long 

 and rising in a ridge, so that in the interbrachial space is a wide angle, and in the 

 brachial space a deep trough. Arm bones long and cylindrical, with only a faint upper 

 furrow. Genital scales long, slender, and cylindrical. 



This genus by its large first under arm plate has a slight relation to Ophioglypha, 

 and by its large tentacle pores at base of arm, an appearance like Op>hiomusium, but its 

 structure is really quite peculiar. 



It has slender connection with any of its neighbours (unless perhaps with Ophio- 

 pleura). The disk covering, though thin and translucent is really composed of rounded 



