12 THE VOYAGE OF H.M.S. CHALLENGER. 
very striking feature when this form is compared with Pararchaster semisquamatus. The 
spinulation of the actinal surface is rather short and robust, and the spinelets which form 
the furrow series on the adambulacral plates are distinctly more cylindrical and robust, and 
show less tendency to radiate apart. The tube-feet have large, well-developed, button-like 
knobs at their distal extremity. The madreporiform body is large and oval, with much- 
convoluted striations. 
Although this and the preceding species conform so closely in their general structure, 
the variations mentioned above, although slight when taken singly, seem to constitute an 
assemblage of characters which mark off this form specifically from Pararchaster semi- 
squamatus. This view is strengthened by the fact that the characters are either indicated 
or present in the young form. 
Young Phase.—A small example measuring only R= 20 mm., 7 = 4 mm., may be 
referred without hesitation to this species, so well are several of the characters noticed 
above as distinguishing the species indicated even at this early stage. The disk is slightly 
inflated, especially in the radial regions, and there are not more than about a dozen of the 
large spines in the central area, belonging to the primary apical plates. One or two small 
thornlets accompany the supero-marginal spine, although the largest of these latter is not 
more than 1°5 mm. in length. On the infero-marginal plates there is as yet only one true 
infero-marginal spine, but on each side of this and slightly below, there is regularly 
present a short thorn-like denticle or spinelet, and there is usually another above or beside 
it on the aboral side. The adambulacral plates have three spinelets on the furrow margin 
(occasionally four near the mouth), and two large spines on their actinal surface. The 
tube-feet have a large and well-developed terminal knob-like extremity. No actinal inter- 
mediate (ventral) plates are present. ‘The madreporiform body is small, subcircular, slightly 
convex, and situated nearly midway between the centre of the disk and the margin : its 
surface is deeply fissured by a few coarse, convoluted striations. The anal aperture is distinct 
and a little excentral, z.e., to the side of the dorso-central plate. Three moderate-sized - 
spinelets stand round its margin. I have not been able to detect any papulz in this example. 
Colour in alcohol, a bleached yellowish white, with a slight brownish or warm ochre 
shade on the abactinal area of the disk. 
Locality.—Station 153. In the Southern Ocean, amongst the pack ice, close to the 
Antarctic Circle. February 14,1874. Lat. 65° 42/0” S., long. 79° 49’ 0” E. Depth 1675 
fathoms. Blue mud. Surface temperature 29°°5. Fahr. This was the most southern 
dredging station during the expedition. 
3. Pararchaster spinosissimus, n. sp. (Pl. I. figs. 1 and 2; Pl. IV. figs. 1 and 2). 
Rays five R= 66 mm.;r=7:5—8 mm. R >8r. Breadth of a ray near the 
base, 7°5 mm, 
