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



plates, is allied to Ophioscolex, but differs by its granulation and its extremely slender, 

 knotted arms. 



Tt is a low genus, and, like O^yhiogeron, has the embryonic arm bone nearly divided 

 into its halves, a condition more fully exemplified in Ophiotholia. 



Ophiosciasma attenuatum, Lym. (PI. X. fig. 13, PL XVI. figs. 1-3). 



Ophioseiasma attenuahmi, Lym., Bull. Mus. Comp. Zool., vol. v., pt. 7, p. 160, pi. vii. figs. 193-195. 



Disk beset with a very fine but not continuous granulation. Arms very slender, 

 knotted, and at centre of the joints diaphanous. Three small, nearly equal arm spines. 



(Type specimen from Station 122.) Diameter of disk 6 mm. Length of arm 45 mm. 

 Width of arm without spines 6 mm Mouth papillae, teeth, and tooth papillae combined 

 in form of a clump of seven or eight sharp thorns or spines, standing round the apex 

 made by the juncture of the long narrow mouth frames. Mouth shields of a transverse 

 diamond shape, very small, sometimes scarcely visible, more or less hidden by thick 

 skin. Side mouth shields very narrow and slender, meeting within. Under arm plates 

 very thin, and in their central part transparent, so that then- outlines are vague, longer 

 than broad, with outer side slightly curved and lateral sides re-enteringiy curved ; length 

 to breadth, '5 : "4. Side arm plates reduced to a strip, like a double cord, running 

 along the side of the joint. No upper arm plates. The arm bones are but slightly 

 calcified, except at their thickened ends, so that theii- more slender central shaft is trans- 

 lucent. The arm thus presents a beaded appearance, with swellings where the ends of 

 the arm bones form joints and support the arm spines. Disk covered with a thick skin, 

 which above is beset with fine grains, and which partly obscures the mouth papillae 

 and the arm spines. No radial shields or disk scales visible. Three small, blunt, nearly 

 equal arm s^^ines, not so long as an arm joint, seen to be microscopically rough when free 

 of skin, and standing nearly at right angles to arm. Tentacles large, simple, and fleshy ; 

 no tentacle scales. Colour in alcohol very pale yellow. 



The animal is covered by a thick, translucent skin, which passes also over the arm 

 spines. There are six slender, knotted arms, four larger and two smaller. 



Station 122.— September 10, 1873; lat. 9° 5' S. to 9° 10' S., long. 34° 49' W. to 

 34° 53' W.; 350 fathoms; mud. 



Ophiogeron. 



OpMogeron, Lym., Bull. Mus. Comp. Zool., vol. v., part 7, 1878. 



Disk covered by a naked skin. Mouth angles naked, except a few small teeth on 



jaw plate. Under arm plates small, and with a large tentacle pore on either side. Side 



arm plates somewhat flaring, and carrying thorny arm spines covered with skin. No 



upper arm plates. 



This genus is somewhat allied with Ophiomyxa and with Ophioscolex. 



