84 



THE VOYAGE OF H.M.S. CHALLENGER. 



with the neighbouring parts. No tentacle pores beyond the basal arm -joints. Small 

 arm spines on outer edge of arm plates. Two genital openings in each interbrachial 

 space. 



In the nature of its covering, this singular genus has some affinity with Ophiolepis, 

 as now restricted. But it is almost unique in having no tentacle pores on the greater 

 part of the arm. 



Seen from within the disk is heavily plated, and has radial shields so large and wide 

 as nearly or quite to make a complete circle round the disk margin. The genital plate 

 and scale are invariably stout and massive, and the latter is so large in Ophiomusium 

 pulchellum as to occupy one-half the lower interbrachial space. In Ophiomusium flabellum 

 where the lower interbrachial space is occupied by the opposite side arm plates, the 

 structure of the genital plate and scale is not known. The mouth frames are flaring and 

 the jaws elongated, so as to occupy an unusual proportion of the disk. There may be 

 either an elementary peristomial plate divided in two parts, or none at all. The arm 

 bones, already somewhat elongated within the disk, are still more so just outside of it, 

 and have a central constriction which gives them, in profile, somewhat the outline of a 

 dice-box. 



See Plate XXXIX. figs. 10-13. 



Table of Species of Ophiomusium. 



' Disk scales numerous and rather flat, with a large \ 



one on the margin. Two or three short, stout > Ophiomusium eburneum. 

 arm spines. Side mouth shields long and large, J 



Interbrachial space between radial shields occupied ' 



by two large plates. Papillae along genital I OMommium sermtum _ 

 openings extending upward as a small arm ' 

 comb. Four or five small arm spines, 



Arms rather slender and taper- 

 ing ; cylindrical, or but 

 slightly wavy from the 

 gentle swelling of the side 

 arm plates. Upper and 

 under arm plates persisting 

 nearly to end of arm ; but * 

 no tentacles beyond third 

 or fourth under plate. 

 Mouth shields small. 

 Genital opening bounded 

 by small close-set papillae. 



Papillae on genital openings extending upward as 

 a small arm comb. Side mouth shields small 

 and short. Upper arm plates comparatively \ Ophiomusium armigerum. 

 large, some of the basal ones inclining to hex- 

 agonal Three or four small arm spines, . J 



Disk thick. Side mouth shields and under arm "j 

 plates much swollen. Outer mouth papilke 

 much larger than inner. Genital papillae large 

 and bead-like. Six or seven small arm spines. ) 



Disk flat, covered with imbricated scales, which "J 

 are nearly equal, except the marginal, which ( q d j,i 

 Upper arm plates large, the basal ( " ' 



\0pli 



kiomusium corticosum. 



are larger. 



ones long hexagonal. 



hiomusium cancellation. 





Disk covered by thick skin : its upper surface 

 occupied chiefly (and in half grown specimens 

 almost entirely), by radial shields and large \ 0phiomusium archast er. 

 primary plates, with small scales between. 

 Arms even and cylindrical. Four minute 

 rudimentary arm spines, 



