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



high as wide, rising in a steep dome surmounted by a sugar-loaf projection, which is the 

 central primary plate, and is surrounded by five other primaries, longer than broad, and 

 hexagonal, outside which are small, angular, radial shields, which are joined ; in the 

 interbrachial space, are one pentagonal and one rounded hexagonal plate, the latter 

 joining the upper edge of the mouth shield ; all the plates are closely soldered, slightly 

 swollen, and, as well as the arm plates, are microscopically tuberculous. The only genital 

 openings seem to be at the inner end of the mouth shields. One minute peg-like arm 

 spine, which is found only on the first two or three joints beyond disk, and stands half- 

 way up the side arm plate. The second pair of mouth tentacles protrude from very large 

 oval pores covered with four flat rounded scales ; the next pores, those of the first arm 

 tentacles, are similar but smaller ; the next still smaller, while those of the fourth under 

 arm plate are little round holes covered by a single scale. Farther out on arm the pores 

 lie behind and above the rudimentary under arm plate. Colour in alcohol, white. 



The large side arm plates and few disk plates suggest that this is a young animal. 

 Unfortunately the depth is given as 18 and 240 fathoms ; but the character of the animal 

 seems to indicate the latter depth. 



Station 172. — July 22, 1874 ; off Nukualofa, Tongatabu ; 18 and 240 fathoms ; coral, 

 sand. 



Ophioglypha. 



Ojohioghjplia, Lym. Proc. Bost. Soc. Nat. Hist, vol. vii., 18G0. 



Disk covered with plates, or scales, which are usually swollen. Eadial shields naked 

 and swollen. Teeth. No tooth papdlse. Mouth papillae long within, but small and 

 short near the outer end of the mouth slit, and partly hidden by the scales of the mouth 

 tentacles. Arm spmes smooth and short, rarely exceeding the length of a joint. 

 Tentacle scales numerous ; the innermost pair of tentacle pores shaped like slits, sur- 

 rounded by numerous tentacle scales, and opening diagonally into the mouth slits. In 

 the back of the disk, where the arm joins it, a notch usually edged with papillae. Two 

 genital slits starting from the sides of the mouth shields. 



Seen from within the upper disk is covered by coarse, irregular, overlapping scales, 

 or plates, and very large, swollen radial shields, which are joined to stout club-ended 

 genital plates whose shaft is flattened and rounded. The genital scale is usually wide, 

 especially at its outer end, which bears the comb and often overlaps the head of the 

 genital plate. In Ophioglypha hexactis, however, and probably in other species which 

 have no arm comb, it is long and narrow. The jaws and mouth frames are stout and 

 have large sockets for mouth tentacles, and a nerve ring more or less exposed by reason 

 of the small size of the peristomial plates, which are sometimes represented only by a 

 thin lime crust, and are sometimes nearly or quite wanting. On their outer and inner 

 faces the arm bones present the typical structure, except that they are as high as, or 



