252 the voyage of h.m.s. challenger. 



Seen in profile from its brachial side, a mouth angle has the look of a wedge-shaped 

 block, having, however, a correspondence with the same part among Ophiurans. Outside 

 is the furrowed articulation of the mouth frame to its fellow (fig 23, x). Then comes, 

 on the upper surface, the peristomial plate (r), followed by the jaw (c), and the jaw plate 

 (e) divided into nodules. On the sides are the great cavities for the first and 

 second mouth tentacles r', ■>■", which have notches in their outer margins for the 

 passage doubtless of their nerves. Just inside the articulating surface (x) are two curved, 

 nearly vertical furrows, whereof the innermost must be for the radial nerve, and the 

 outermost for the radial water tube. Among the terminal arm twigs, the structure of 

 side arm plates, hook grains and tentacle hooks is essentially that of Gorgonocephalus 

 (figs. 19, 20, 22) ; farther inward, however, all hooks, whether on grains or on side arm 

 plates, disappear, a fine granulation covers the arm (fig. 21), and even the tentacle pores 

 themselves are obliterated inside the second fork (fig. 17). From both other genera, the 

 present is distinguished by a total absence of under arm plates, unless the little plate at 

 the outer corner of the mouth slit may be called one (fig. 17, h). On the other hand the 

 side arm plates are strongly developed although confined, as usual, to the under surface 

 and lower sides of the arm. Near the tip they have the usual shape (fig. 20, i) ; but 

 near the base they take on the form of wide flat plates, meeting on the median line, and 

 having small re-entering curves on their inner and outer edges, whereby little vacant 

 ovals are left which look like very large tentacle pores (fig. 17, i). Already on the 

 smaller twigs the tentacle hooks on the side arm plates (fig. 22, q) have changed from a 

 curved and sharp outline (fig. 20, q) to a blunt spine-like form ; and soon after, they 

 drop off ; a fine granulation covers the arm, and within half a dozen forks of the tip, 

 nothing appears of the side arm plate but a small mamelon with a tentacle hole 

 (fig. 21, i). An examination of the under side of an arm bone shows that the tentacles 

 do not, as Gaudry supposed, lie between the bones, but on the front under surface 

 (fig. 26, r) quite as among Ophiurans. At the bottom of the tentacle socket is a pore 

 for the water tube which should first pass into the hole above and nearer the centre, and 

 so curve upward through the substance of the bone, to descend again to the tentacle socket. 

 The numerous specimens of Gorgonocephalus, and their different sizes furnish material 

 for a sketch of the growth of the hard parts. The young, with a disk 2 - 5 mm. in 

 diameter (PI. XXXVI. figs. 2, 3), is covered with a skin which, when dry, exhibits 

 distinctly the underlying plates somewhat like those of Opltiolepis. The jaw (c) with 

 its teeth (cV) are joined to two large side mouth shields. From this point of view no 

 jaw plate can be seen till the animal is larger (fig. 17, e). Outside the side mouth 

 shields is a plate which holds the position of a mouth shield, and sometimes takes the 

 function of a madreporic plate (fig. 17, a). The remainder of the lower interbrachial 

 space is covered by eight or nine irregular plates. Above, there is in the centre a group 

 of six or seven primary plates(fig. 3, g), each encircled by a superimposed line of grains. 



