REPORT ON THE OPHIUROIDEA. 81 



1*2: 2*6. Side mouth shields long and narrow, extending along the inner angle of the 

 mouth shield, and just meeting within, where they broaden slightly, becoming narrower 

 at the outer end where attached to the side arm plate. First under arm plate much larger 

 than the others, four sided, with outer edge straight, lateral sides re-enteringly curved, 

 and a short side, like a truncated angle within ; the rest of the under arm plates are very 

 small, with a curved outer side, and are nearly semicircular in form, but have a slight 

 peak inward at the junction of the side arm plates, which are thin and wide, flaring 

 slightly outward, meeting broadly below, and separated above by the upper arm plates, 

 which are four sided, with outer edge nearly straight and wider than inner. Disk round, 

 smooth, and flat, covered with minute, very thin, overlapping scales, four or five in the 

 length of a millimetre on upper surface, and more below ; among them may be distinguished 

 small, round, widely separated primary plates. Radial shields longer than broad, three 

 sided, with all three angles much rounded, widely separated their entire length by the 

 disk scaling; length to breadth 2'6 : 1*5. Genital scales hidden, except the outer tip, 

 which shows outside the radial shield, and bears no papillae, or only one or two rudi- 

 mentary. Three sharp, slender, tapering arm spines, situated close to each other on the 

 middle edge of the side arm plate. Only one very minute, rounded tentacle scale on the 

 outer edge of some of the tentacle pores ; a little way out on the arm there is usually a 

 minute scale on the inside of the pore, and none on the outer side. Colour in alcohol, 

 pale straw. 



Beyond the disk there is almost always a tentacle scale, which is often lacking in the 

 basal joints. The little papilla outside the pore is also frequently wanting. Some speci- 

 mens have a small arm comb, and even a feeble one on the basal upper arm plate. 



Station 156.— February 26, 1874; lat. 62° 26' S., long. 95° 44' E. ; 1975 fathoms; 

 diatom ooze. Station 160.— March 13, 1874; lat. 42° 42' S., long. 134° 10' E. ; 2600 

 fathoms ; red clay. 



Ophiocten umbraticum, Lym. (PL IX. figs. 1-3). 



Ophiocten umbraticum, Lym., Bull Mus. Comp. Zool., vol v., part 7, p. 101, pi. v. figs. 131, 

 132, 1878. 



Primary and marginal disk plates small, and the former widely separated. Arm 

 comb feeble. Tentacle scales single and minute, or quite wanting. 



(Type specimen from Station 325.) Diameter of disk 9"5 mm. Width of arm close 

 to disk, without spines, 1*3 mm. Four rather wide, closely -joined mouth papillae, with 

 one larger, diamond-shaped, at apex of the jaw. Mouth shield, very thin, triangular, 

 broader than long, with blunt angle inward and corners rounded; length to breadth 

 1 : 1*5. Side mouth shields long and narrow, tapering inward, where they nearly or quite 

 meet. First under arm plate large, four sided, the inner side being very short like a 



(zool. CHALL. EXP. — PART XIV. — 1882.) Oil 



