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



whereof the one next the under arm plate is much the smaller. Colour in alcohol 

 pale grey. 



Station 171.— July 15, 1874 ; Lit, 28° 33' S., long. 177° 50' W. ; GOO fathoms; rock. 



The following four genera are allied. Ophiacantha is distinguished by the delicate 

 scales and slender radial shields obscured by skin ; Ophiothamnus, by its wide naked 

 radial shields, and by the peculiar jaw cover of three symmetrical pieces ; Ophiomitra by 

 the coarse disk plates and scales, and wide naked radial shields ; and Ophiocamax by 

 the tufts of numerous spiniform mouth and tooth papillae, and the scales of the mouth 

 tentacles borne on a special plate. The old genus Ophiacantha embraces the greatest 

 variety of forms, from Ophiacantha bidentata and Ophiocantha vivipara, which come 

 near Ophiocoma, to the delicate Ophiocoma pentacrinus, with its thorny, translucent 

 arm spines and its knotted arms. 



Ophiacantha. 



Ophiacantha, Mull. & Tr. Syst. Ast., 1842. 



Disk somewhat thick and swollen and clad in a thin skin, that more or less obscures 

 the underlying even coat of fine imbricated scales, which sometimes completely covers the 

 long narrow radial shields, and bears spines, thorns, or rough grains. No tooth papillse. 

 Mouth angle rather large and bearing numerous (7-16) sharp, rather long, papillae. 

 Teeth sharp and elongated. Arm spines hollow, numerous (4-11); usually rough or 

 thorny. Side arm plates large, and nearly or quite meeting above and below. Two 

 genital openings in each brachial space. 



From its under side the disk scaling is seen to be thin and usually uniform. It is 

 supported by long narrow, bar-like radial shields, whose outer end is but little enlarged 

 where it joins the thick, club-headed, somewhat rounded genital plate, to which is 

 attached a short, blade-like scale. The arm bones are wider than high, and have 

 thin simple wings whose margins are not grooved. Their outer and inner faces are of 

 the typical form, and have the lower canal like a small, nearly-closed notch. The 

 mouth angles are stout and compact, and the tops of the mouth frames wide, a 

 portion being covered by the thick, rounded, rather large peristomial plate, which is 

 in a single piece. 



A comparison of species shows some good specific differences resting on internal 

 structure, and some divergence from the generic type. Thus Ophiacantha abnormis 

 has both jaw and peristomial plate exceptionally narrow, while the radial shield is wide 

 and thin, and the genital scale much longer than is usual. Ophiacantha vivipara and 

 Ophiacantha granulosa have their radial shields prolonged inward by lines of imbricated, 

 supplementary scales, thus showing a slight approach to Ophiocoma. 



See Plate XLI. figs. 12-14. 



