18 . THE VOYAGE OF H.M.S. CHALLENGER. 
adambulacral plates, widely spaced and standing in a subregular lineal series parallel to 
the median suture. On the anterior part of the plate are two or three isolated and much 
smaller spinelets, and there is usually a thornlet or miliary spinelet opposite the middle 
spine of the three main superficial mouth-spines, or opposite the interspace between the 
middle and the outermost. The surface of the plates is covered with membrane and no 
other spinelets are present. 
The actinal interradial areas are very small, and do not extend beyond the third or 
fourth marginal plate, and probably not more than twelve or fifteen intermediate plates 
are present in each; but the exact number is undeterminable on account of the whole area 
being covered with membrane. Between each of the plates of the inner row is one of the 
spiracle-like pedicellarian apparatus, similar to those just described, but much larger ; 
and there are thus five or six in each area, The few remaining intermediate or ventral 
plates bear one or two short, cylindrical, obtusely tipped spinelets. 
The anal aperture is subcentral, small and very distinct; it is surrounded by small 
spinelets somewhat larger than the small spinelets on the spicules generally. 
The madreporiform body which is comparatively large and oval, is situated a little on 
the outer side of midway between the centre of the disk and the margin. Its surface is 
sub-plane or very faintly convex, and is grooved with highly convoluted furrows, the 
general direction of the striations appearing to radiate from a central point to the margin. 
The papulz, which are rather large and distinct, are confined to an area at the base 
of the rays, which does not extend beyond the fourth supero-marginal plate, but reaches 
on the disk as far as the spine referred to as marking the primary radial plate. No 
papule are present on a band-like area along the median interradial line; and on the 
outer part of the papularium at the base of the ray, there is likewise a V-shaped area, 
with the angle passing far downward adcentrally along the median line, which is similarly 
devoid of papule. 
Colour in alcohol, a bleached ashy white ; with some traces of a brownish, or dark 
purple, pigment remaining upon the abactinal surface. 
Individual Variation.—In some cases the small spinelets on the abactinal plates 
or spicules are very regularly grouped into threes or fours along the ray, excepting just at 
the sides. The regularity of this arrangement gives at first sight rather a striking feature. 
In other examples no such general grouping takes place, although here and there a faint 
approach to it may be noticed. The usual pedicellarise are present in both instances. 
The second large spine on the infero-marginal plates is more definitely developed in some 
cases than others; and in some specimens when this spine is small its character is in a 
great degree masked by the presence of a companion smaller spinelet standing beside 
it, giving the appearance of a pair of small spinelets standing side by side below the true 
lateral spine. 
Young Phase (?).—There is a very small specimen from Station 143 which is probably 
