32 THE VOYAGE OF II.M.S. CHALLENGER. 



parallel to the median suture, which decrease in size as they proceed outward ; and 

 between this series and the outer margin stand two small secondary spines, or sometimes 

 only one, accompanied by one or two miliary spines ; the secondary spines are, however, 

 so small that they are scarcely distinguishable from the rest of the spinulation. 



The actinal intermediate (ventral) plates are very few in number, not more than six to 

 twelve in each area. They are small and convex, and bear two or three small tapering 

 thorn-like miliary spinelets on their midst. They do not separate more than the innermost 

 three or four adambulacral plates from the infero-marginal plates. 



The anal aperture is subcentral and often difficult to see, but its position is usually 

 indicated by the crowded circlet of paxillae around it, although these differ in no way from 

 the rest. 



The papularium at the base of each ray is small and compact, probably with not more 

 than six to nine papulae in each, and often, I believe, with a less number ; but this is 

 difficult to ascertain without dissection. The size of the papulae varies considerably in one 

 and the same papularium, one or two being often much larger than the others. 



The madreporiform body, which is small, oval, and subtubercular, is situated about 

 midway between the centre and the margin, but rather nearer the latter. Its surface is 

 marked with deep, coarse, convoluted furrows, which usually have the appearance of 

 traversing the plate rather than of radiating from the centre to the margin ; and there is 

 frequently a large paxilla on its adcentral side. 



Colour in alcohol, a dirty yellowish ashy grey. 



Individual Variation. — This species is on the whole very constant in character ; the 

 only variations of any moment, which I notice in a considerable number of examples all 

 from one station, affect the relative size and robustness of the prominent or largest spine- 

 lets on the actinal surface of the adambulacral plates ; and the degree of development of 

 the small companion to the lateral spine on the infero-marginal plates. There is also 

 sometimes a slight variation — diminution — in the size of the minute miliary thornlets 

 or papillse on the actinal intermediate and infero-marginal plates. It may also be 

 remarked in some examples that there is a well-defined plate or paxilla much larger than 

 any of the others midway between the centre of the disk and the margin in the median 

 interradial line, and these are probably the representatives of the primary interradial 

 (basal) plates. 



Young Phase. — The smallest example of Pontaster planeta in the collection measures 

 R = 15 5 mm. and r = 4 "5 mm. It presents all the characters of the adult in an unmistak- 

 able way, and there would be no hesitation in referring it to the species even if inter- 

 mediate stages of growth were wanting. The relative shortness of the radial dimension in 

 proportion to the interradial may be noticed. There are sixteen supero-marginal plates 

 between the terminal and the median interradial line. The terminal plate is large and 

 broad, and deeply indented proximally in the median line. 



