12 THE VOYAGE OF ILM.S. CHALLENGER. 



very striking feature when this form is compared with Pararchaster semisquarnatus. 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 striatums. 



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- 

 squarnatus. 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., r = 4 mm., may be 

 referred without hesitation to this species, so web 1 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-margiual 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-Hke 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-bike 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, i.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 papulae 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. (PI. I. figs. 1 and 2 ; PI. 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. 



